Add tests + better docs for FactoredMatrix

[neelnanda-io]

Add tests that the FactoredMatrix class works (essentially that each of its methods correctly mimics the result for the actual matrix product).

Go through the documentation and clarify things that are unclear (this is hard for me to do, so even just having someone new to the library flag confusions is helpful!)

[inwaves]

Taking a look at this over the next week or so.

[rusheb]

Hello, I would like to help with adding unit tests. I may find it hard to make all the changes all at once, and it would be good to work incrementally to make sure I am on the right track!

Are you okay with me breaking down the work into multiple PRs? This may also enable the work to be parallelised between several developers.

If so, the following table could be used to track progress:

method	pull request	assigned	merged	notes
__init__	https://github.com/neelnanda-io/TransformerLens/pull/188	@rusheb	✅
__matmul__		@rusheb
__rmatmul__
AB
BA
T
svd				includes U, S and Vh properties
eigenvalues
__getitem__
norm
__repr__
make_even
get_corner
collapse_l
collapse_r
unsqueeze
pair

[rusheb]

I think that all the tests for this are actually done as of #218. @jbloomAus maybe we should close this ticket and spin out a new one for docs.