Refactor expectation_value implementations

[leburgel]

Some refactoring and reorganization related to the evaluation of expectation values. The main motivation for this refactor is making it easier to add support for computing expectation values of different kinds of operators, and evaluate these using different kinds of environments. Some examples of this are given in #425 and #426, which are built on top of this refactor.

Short summary:

• Moved around some existing code to different/new files, hopefully improving the organization a bit.
• Remove as many type annotations as possible from the methods used along the evaluation of expectation values, allowing to reuse the main entry points for different types of operators and environments.
• Insert a dedicated local_expectation_value method to catch local expectation values for different kinds of operators. This goes through the reduced_densitymatrix approach for AbstractTensorMap local terms, but can follow a different path for different kinds of local terms (see for example [WIP] Add support for local expectation values of tensor product and MPO terms #425).
• Implement BPEnv expectation values via reduced density matrix contraction along the generic AbstractTensorMap path, by specializing the appropriate methods.
• Write the main local patch contraction as generic methods in terms of a fixed set of contraction expression generators . These generators then dispatch on the appropriate type to generate an appropriate expression. The idea would be that to add support support for a different type of state, operator or environment, we now only need to overload one of the main expression generators boundary_contraction_expr, bulk_contraction_expr or operator_contraction_expr.

[codecov]

Codecov Report

❌ Patch coverage is 94.89796% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/utility/util.jl	75.00%	4 Missing ⚠️
src/algorithms/contractions/bp_contractions.jl	93.75%	1 Missing ⚠️

Files with missing lines	Coverage Δ
...hms/contractions/ctmrg/characteristic_equations.jl	87.59% <ø> (+0.46%)	⬆️
src/algorithms/contractions/ctmrg/network_value.jl	100.00% <100.00%> (ø)
src/algorithms/contractions/localoperator.jl	99.19% <100.00%> (+2.73%)	⬆️
src/algorithms/ctmrg/gaugefix.jl	93.57% <ø> (ø)
src/algorithms/toolbox.jl	95.19% <100.00%> (-1.74%)	⬇️
src/algorithms/contractions/bp_contractions.jl	90.32% <93.75%> (-2.06%)	⬇️
src/utility/util.jl	76.14% <75.00%> (-0.20%)	⬇️

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[Yue-Zhengyuan]

I'll try coming up with a source tree of expectation-value-related functions to better discuss where each function should be placed.