Iterable states

[ziofil]

Context:
Adding a new feature to iterate over the marginals of a multimode state.

Description of the Change:
With this PR we support the syntax (e.g.)

for mode in state:
    print(mode.purity)

Benefits:
cleaner syntax.

Possible Drawbacks:
maybe one could expect that for xxx in state yields something other than the single-mode marginals.

Related GitHub Issues:
None

[codecov]

Codecov Report

Merging #140 (616cc21) into main (bc4ae38) will increase coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #140      +/-   ##
==========================================
+ Coverage   71.22%   71.24%   +0.02%     
==========================================
  Files          29       29              
  Lines        2811     2813       +2     
==========================================
+ Hits         2002     2004       +2     
  Misses        809      809              

Impacted Files	Coverage Δ
mrmustard/lab/abstract/state.py	74.52% <100.00%> (+0.19%)	⬆️

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[ziofil]

Oh I still need to update the changelog