Add rank and cond

[lkdvos]

I added the utility rank and cond functions, since I thought this would help fix #219 . I overlooked that there S is already a vector, so that actually didn't help, but it might be useful anyways?

To do

• Add tests

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.51%. Comparing base (511fa90) to head (1c2dd61).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/tensors/linalg.jl	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   82.44%   82.51%   +0.06%     
==========================================
  Files          43       43              
  Lines        5536     5552      +16     
==========================================
+ Hits         4564     4581      +17     
+ Misses        972      971       -1     

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[Jutho]

Looks good to me. Is the "add tests" an actual todo?

[lkdvos]

What is an "actual" to do? I would definitely say "should do"...

[Jutho]

I am making some changes (the condition number of the map on an empty space should be one, not zero. Condition numbers lie in the range [1, +inf]). But now for the difficult question: should rank count the degeneracy, i.e. is it sum(dim(c) * rank(b)) or simply sum(rank(b)) ?

[lkdvos]

With regards to the condition number, I took this from LinearAlgebra, see also this discussion and the related MATLAB discussion. I have no opinion about this I think, whatever you feel is most consistent is good to me.

For the rank, I guess if the matrix is full rank, you would expect rank(A) = dim(domain(A)), so whatever is consistent with that?

[Jutho]

Ok I see regarding condition number. Not my favorite pick, but I can live with it for consistency.

For the rank, that choice would include quantum dimensions, which I am definitely happy with as we also do this in other places.

[Jutho]

@lkdvos , if the tests pass, this is good to go for me.

[lkdvos]

Nightly test failures are unrelated, so I'm merging this now.