Adapt python wrappings

This MR updates the dependencies for the pybind bindings of the algorithms image_io video_input and metadata_map_io . Instead of also bringing the pybindings of these classes we introduce ./script/cpp_to_pybind11.py that generates those bindings. The script was built so that the generated code closely resembles that of the kwiver-v1 taken into account the differences in algorithm API in kwiver-v2. It also has the advantage of bringing the documentation of the classes from C++ which was previously missing.

Documentation example

>> help(ffmpeg_video_reader)
Help on VideoInput in module kwiver.vital.algo._algos object:

class VideoInput(_algorithm)
 |  Method resolution order:
 |      VideoInput
 |      _algorithm
 |      pybind11_builtins.pybind11_object
 |      builtins.object
 |  
 |  Methods defined here:
 |  
 |  __init__(...)
 |      __init__(self: kwiver.vital.algo._algos.VideoInput) -> None
 |  
 |  close(...)
 |      close(self: kwiver.vital.algo._algos.VideoInput) -> None
 |      
 |      \brief Close video stream.
 |      
 |      Close the currently opened stream and release resources.  Closing
 |      a stream that is already closed does not cause a problem.
...

A new test was brought from kwiver-v1 python/kwiver/vital/tests/test_image_io.py that validates that bindings work for image_io and that we can create a new image_io implementation from the python side.

Ideally we would like to also bring https://github.com/Kitware/kwiver/blob/master/python/kwiver/vital/tests/test_algorithms_common.py but the we would need to update the test for a newer pytest since it looks like some tests are skipped: PytestCollectionWarning: yield tests were removed in pytest 4.0 - test_XXX will be ignored We will do this in a separate MR.

IMPORTING_GUIDE.md on https://hackmd.io/@9L74IuqXTpe4t4y1HbN-XQ/HywL9UW9p/edit is also updated.