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).