Add tests of .mat file conversion and extraction

[danielolsen]

Copied from #79

To improve the test coverage [of matfile extraction], I think the ultimate thing to do is to:

• Use Julia (via python) to write a) the input.mat file, and b) a set of result_*.mat files in the way that REISE.jl writes them now.
• Test that we can adequately interpret and convert the matfile v7.3 input.mat file and save to a scipy-compatible v7 *_grid.mat file.
• Test that we can adequately interpret and process the result_*.mat files into .pkl files (make sure that we get the standard files plus make sure that we get additional files if storage and/or load shedding are present)