Add Seg3D specific PYTHONPATH environment variable

[a-y-khan]

Add support (and documentation) for reading a Seg3D specific PYTHONPATH environment variable, something like SEG3D_PYTHONPATH, so it won't interfere with the general PYTHONPATH for system python installations. It should be added to the interpreter paths during initialization. See also SCIRun issue 1252.

[a-y-khan]

Py_IgnoreEnvironmentFlag is currently on.