Add custom integration paths and callbacks to josephson and densitymatrix

[pablosanjose]

With this PR we can give a collection of complex ωpoints to josephson(gs, ωpoints;...) and densitymatrix(gs, ωpoints; ...) and the integration will obediently be done along a polygonal path connecting them, without any sawtooth added. Only if ωpoints are all real will a sawtooth of the given slope be created.

We also added a callback kwarg that can be used to perform any operation at each integration step.

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Codecov Report

Attention: Patch coverage is 93.61702% with 6 lines in your changes missing coverage. Please review.

Project coverage is 93.05%. Comparing base (365c267) to head (278fdae).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/observables.jl	93.18%	6 Missing ⚠️

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@@            Coverage Diff             @@
##           master     #307      +/-   ##
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+ Coverage   92.77%   93.05%   +0.27%     
==========================================
  Files          38       38              
  Lines        6494     6548      +54     
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+ Hits         6025     6093      +68     
+ Misses        469      455      -14     

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[pablosanjose]

We add here some extra features and fixes:

• We move omegamap from Integrator to the integrand of densitymatrix and josephson, so that Quantica.integrand(x) returns an integrand object that properly applies omegamap at each point.
• We add a Quantica.path(x) for x::Josephson{<:JosephsonIntegratorSolver} and x::DensityMatrix{<:DensityMatrixIntegratorSolver} that gives the vertices that will be used for integration.
• Allow ωpoints to be any type of container. We check statically whether all elements are real to apply a sawtooth transformation. Otherwise, we use them untouched.