Use permutation matrices where relevant

[James Lucas]

We should replace the existing permutation matrix hacks throughout the library with the new PermutationMatrix types. Relevant locations:

• LUP decomposition
• Parity function in matrix/mod.rs

[Andreas Borgen Longva]

Just a heads-up in case anyone would want to work on this: please let me know, and we can work together! I'm hoping to be able to come up with a second draft of #94 soon. It will be completely rewritten and rely on the new PermutationMatrix struct.

[James Lucas]

I think this has been completed now, @Andlon ?

[Andreas Borgen Longva]

I would agree :)