Incorporate mat73

[ecarl65]

This package is great! It provides something so much more usable than the default scipy.io.loadmat. But it does seem to struggle on a lot of newer matlab formats. I noticed there's a mat73 project that works great for reading the HDF5 format for .mat files in version 7.3. But it doesn't write them. At any rate, it would be nice if this project could be updated to handle newer formats and the effort not duplicated too much.

https://github.com/skjerns/mat7.3