Python3 compatible scripts

Change use of python in ParaView to be Python3 compatible.

The most common changes:

• print "foo" => print ("foo")
• raise RuntimeError, "failed" => raise RuntimeError("failed")
• next() => __next__()
• import of relative submodules is disallowed.
• use import __future__ to get python3 behavior in python2

Spot fixes:

• map() returns an iterable, not a list
• type str now has attr __iter__, so looks iterable.
• python3 dropped iteritems() - just use key to get value.

@utkarsh.ayachit any python gurus who should take a look?