Add Quantica.integrand for josephson currents

[pablosanjose]

Provides an unexported method j = integrand(J::Josephson, kBT = 0.0) to obtain the function whose integral over ω yields the Josephson current J(kBT), where J = josephson(...). The integrand can be called with Quantica.call!(j, ω; params...) (mutating) of j(ω; params...) (allocating)

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Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.77%. Comparing base (85edbfc) to head (6bb8129).

Files with missing lines	Patch %	Lines
src/observables.jl	66.66%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
- Coverage   92.79%   92.77%   -0.02%     
==========================================
  Files          38       38              
  Lines        6492     6494       +2     
==========================================
+ Hits         6024     6025       +1     
- Misses        468      469       +1     

Flag	Coverage Δ
	92.77% <66.66%> (-0.02%)	⬇️

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