Add mat_inverse method

[panxinmiao]

Following @Korijn 's suggestion in pygfx/pygfx#919 (comment), added the mat_inverse method to pylinalg.

[Korijn]

Nice work! Just need to address the merge conflicts and the one review comment, then we can merge. :)

[almarklein]

Nice work! A comment and a question:

• The method option "manual" strikes me as a bit odd. It's not really manual, but pure Python.
• Would it be good to use this new inv method internally? If I checked correctly, we (only) use it in vec_unproject()

[Korijn]

• The method option "manual" strikes me as a bit odd. It's not really manual, but pure Python.
• Would it be good to use this new inv method internally? If I checked correctly, we (only) use it in vec_unproject()

Didn't think about that! I will release a patch.

[Korijn]

Done in #100