mler requires ffmpeg

[ssolson]

Currently, the MLERexportMovie method requires the user to have FFmpeg installed and in the current path. FFmpeg is not listed as a dependency in the documentation or otherwise.

This bug can be invoked if the user does not have FFmpeg installed and runs example_MLER_testrun.py.

We should investigate other methods to create a movie using python to see if this dependency can be avoided. If not we need to add this to the documentation as a dependency.