WIP: Symmetric ordering

[david-pl]

I implemented a simple struct that acts as a QSym avoiding application of commutation relations. This can be used in a simple method for computing the symmetrized equations out of the set of equations that uses normal ordering. As a first test I generalized the example using position and momentum operators from the documentation to higher orders (https://qojulia.github.io/QuantumCumulants.jl/dev/implementation/#interface).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.08%. Comparing base (7b0c103) to head (9debb31).
Report is 169 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   82.76%   83.08%   +0.31%     
==========================================
  Files          15       16       +1     
  Lines        1700     1726      +26     
==========================================
+ Hits         1407     1434      +27     
+ Misses        293      292       -1     

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