settings context manager

[ziofil]

Allows settings to be set locally:

with settings(ATOL = 1e-5):
    # in here settings.ATOL is 1e-5

# here it's what it was previously

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.56%. Comparing base (d0ba778) to head (696b4b9).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #477      +/-   ##
===========================================
+ Coverage    89.55%   89.56%   +0.01%     
===========================================
  Files          101      101              
  Lines         7384     7392       +8     
===========================================
+ Hits          6613     6621       +8     
  Misses         771      771              

Files with missing lines	Coverage Δ
mrmustard/utils/settings.py	100.00% <100.00%> (ø)

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