Added some CMake macros to support discovery of Python modules.

Added a CMake file with two macros that support finding Python
modules on the system in a slightly more efficient way.  One of the
goals is that hereafter we will only need to run the external Python
process once for each module we need to test, and we will cache the
results so that subsequent checks can avoid the additional overhead.

There were two CMakeLists.txt where we were checking for the presence
of matplotlib, and these have been changed to demonstrate how the new
macros will be used.  As soon as these changes are accepted into VTK,
we would like to start using them in a similar fashion from the
paraview web testing framework.

Change-Id: I5439b10a4d8c0f8dc0b4209bbd429b104ef7c9c2
Merge topic 'cmake-python-modules-check' into master

658b5bbb Added some CMake macros to support discovery of Python modules.