Integration path override in densitymatrix and josephson

[pablosanjose]

When creating a ρ = densitymatrix(gs, ωpoints; kw...) or J = josephson(gs, ωpoints; kw...) object using the generic integration algorithm, one specifies the integration path through ωpoints (see #307 ). However, it may be necessary to override the shape of the path as a function of parameters or temperature, for example, which are not known until we actually evaluate the object with ρ(µ, kBT; params...) or J(kBT; params...). For this reason we provide here an advance call syntax

ρ(µ, kBT, override = missing; params...)
J(kBT, override = missing; params...)

where override can be a collection of ωpoints that replace the original ωpoints, or a function that is applied to them before integration. For technical reasons the override syntax is currently only supported if the original ωpoints are not all real (since in that case a different sawtooth transform takes place as per the docstrings)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.03%. Comparing base (3d52e7f) to head (db5011b).

Files with missing lines	Patch %	Lines
src/observables.jl	90.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
- Coverage   93.05%   93.03%   -0.02%     
==========================================
  Files          38       38              
  Lines        6548     6560      +12     
==========================================
+ Hits         6093     6103      +10     
- Misses        455      457       +2     

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