STYLE: Add .flake8 configuration to support python script validation

Flake8 is a python source checker.

The initial configuration file adds exceptions for all errors, it will
iteratively be updated as the code is improved.

After installing flake8 with `pip install flake8`, the following shell
script was used to craft the list of exceptions:

  IFS=$'\n'
  for line in $(flake8 --exclude src/Admin/ . | cut -d":" -f4 | sort | uniq)
  do
     # Remove leading space
     line=$(echo $line | sed -e 's/^[ \t]*//')
     # Extract error code
     code=$(echo $line | cut -d" " -f1)
     echo "    # $line"
     echo "    $code,"
  done

Currently, `src/Admin/` is excluded.

Internally flake8 is wrapper around these tools:

* PyFlakes: analyzes programs and detects various errors. It works by
  parsing the source file, not importing it, so it is safe to use on
  modules with side effects. It’s also much faster.

* pycodestyle: a tool to check your Python code against some of the
  style conventions in PEP 8 (the style guide for python code).
  See https://www.python.org/dev/peps/pep-0008/

* Ned Batchelder’s McCabe script: check McCabe (or cyclomatic) complexity.
  See https://en.wikipedia.org/wiki/Cyclomatic_complexity.

To learn more about flake8. See http://flake8.pycqa.org/en/latest/


Former-commit-id: 6f028716

.flake8

+[flake8]
+exclude =
+    src/Admin/
+ignore =
+    # E111 indentation is not a multiple of four
+    E111,
+    # E114 indentation is not a multiple of four (comment)
+    E114,
+    # E122 continuation line missing indentation or outdented
+    E122,
+    # E123 closing bracket does not match indentation of opening bracket's line
+    E123,
+    # E126 continuation line over-indented for hanging indent
+    E126,
+    # E127 continuation line over-indented for visual indent
+    E127,
+    # E131 continuation line unaligned for hanging indent
+    E131,
+    # E201 whitespace after '('
+    E201,
+    # E201 whitespace after '['
+    E201,
+    # E202 whitespace before ')'
+    E202,
+    # E203 whitespace before ','
+    E203,
+    # E211 whitespace before '('
+    E211,
+    # E221 multiple spaces before operator
+    E221,
+    # E225 missing whitespace around operator
+    E225,
+    # E226 missing whitespace around arithmetic operator
+    E226,
+    # E231 missing whitespace after ','
+    E231,
+    # E241 multiple spaces after '
+    E241,
+    # E241 multiple spaces after ','
+    E241,
+    # E251 unexpected spaces around keyword / parameter equals
+    E251,
+    # E261 at least two spaces before inline comment
+    E261,
+    # E262 inline comment should start with '# '
+    E262,
+    # E265 block comment should start with '# '
+    E265,
+    # E266 too many leading '#' for block comment
+    E266,
+    # E302 expected 2 blank lines, found 1
+    E302,
+    # E303 too many blank lines (2)
+    E303,
+    # E303 too many blank lines (3)
+    E303,
+    # E303 too many blank lines (4)
+    E303,
+    # E305 expected 2 blank lines after class or function definition, found 1
+    E305,
+    # E401 multiple imports on one line
+    E401,
+    # E501 line too long
+    E501,
+    # E701 multiple statements on one line (colon)
+    E701,
+    # E702 multiple statements on one line (semicolon)
+    E702,
+    # E703 statement ends with a semicolon
+    E703,
+    # E721 do not compare types, use 'isinstance()'
+    E721,
+    # E999 SyntaxError
+    E999,
+    # F401 'PyQt4.QtCore' imported but unused
+    F401,
+    # F401 'random' imported but unused
+    F401,
+    # F401 'string' imported but unused
+    F401,
+    # F401 'vtk.*' imported but unused
+    F401,
+    # F403 'from numpy import *' used; unable to detect undefined names
+    F403,
+    # F403 'from vtk import *' used; unable to detect undefined names
+    F403,
+    # F405 'XXXX' may be undefined, or defined from star imports
+    F405,
+    # F821 undefined name 'constant'
+    F821,
+    # F821 undefined name 'os'
+    F821,
+    # F821 undefined name 'unicode'
+    F821,
+    # F841 local variable 'iren_list' is assigned to but never used
+    F841,
+    # F841 local variable 'lcolorName' is assigned to but never used
+    F841,
+    # F841 local variable 'lutUtilities' is assigned to but never used
+    F841,
+    # F841 local variable 'pd' is assigned to but never used
+    F841,
+    # F841 local variable 'ptId' is assigned to but never used
+    F841,
+    # F841 local variable 'timerId' is assigned to but never used
+    F841,
+    # W191 indentation contains tabs
+    W191,
+    # W291 trailing whitespace
+    W291,
+    # W293 blank line contains whitespace
+    W293,
+    # W391 blank line at end of file
+    W391,