Refactor MPOHamiltonian

[lkdvos]

This is the second stage in refactoring the operators.
This update features more support for finite hamiltonians, along with a new way of constructing them from local operators.
The goal is to port over some of the functionality from MPSKitModels, such that that package can become exclusively for defining models.

[codecov]

Codecov Report

Attention: Patch coverage is 81.39535% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 83.25%. Comparing base (3d27cfb) to head (15cb2c5).
Report is 2 commits behind head on master.

Files	Patch %	Lines
src/operators/mpohamiltonian.jl	80.18%	22 Missing ⚠️
src/operators/densempo.jl	66.66%	1 Missing ⚠️
src/states/abstractmps.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
+ Coverage   83.20%   83.25%   +0.04%     
==========================================
  Files          72       72              
  Lines        4585     4705     +120     
==========================================
+ Hits         3815     3917     +102     
- Misses        770      788      +18     

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