Partial evaluation of GreenFunctions

[pablosanjose]

Closes #200

We add support for g´ = g(; param...) with g::GreenFunction. This gives a new GreenFunction that encloses a new type of g´::GreenSolver (of type FixedParamGreenSolver) that acts like a wrapper of the original one, but that overrides params when called. The new g´ is completely decoupled from g (so this is not a lazy wrapper), much like h´ = h(; params...) with h::ParametricHamiltonian is decoupled from h.

(g::GreenSlice)(; params...) is also supported

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 92.52%. Comparing base (55f1659) to head (be7a443).

Files	Patch %	Lines
src/greenfunction.jl	75.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #270      +/-   ##
==========================================
- Coverage   92.54%   92.52%   -0.02%     
==========================================
  Files          37       37              
  Lines        6100     6101       +1     
==========================================
  Hits         5645     5645              
- Misses        455      456       +1     

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