WIP: read (some) v4 format files

[kleinschmidt]

This is based on the spec I found here: https://www.eiscat.se/wp-content/uploads/2016/03/Version-4-MAT-File-Format.pdf It doesn't handle everything but maybe it's better than nothing!

Unfortunately I lack a sufficiently diverse collection of .mat files to test this against but it works for the one type of file I do have a lot of (a single Int16 matrix per .mat file).

[ViralBShah]

@kleinschmidt Apologies for a ping after such a long time, but is it ok to merge this as is?

[kleinschmidt]

Don't see why not, but to be perfectly honest I haven't touched a MAT file in many years

[kleinschmidt]

Ah hold up, there's #186

[matthijscox]

This PR does not seem to add any new features, so let's close it.