Disallow integrand params to be different from integration path params

[pablosanjose]

Closes #329 (implements fix 1 therein)

When the integrand is built using params, these are bundled into a closure, together with omegamap, which simplifies the JosephsonIntegrand and DensityMatrixIntegrand structs.

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Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.44%. Comparing base (f64e499) to head (e516fcd).

Files with missing lines	Patch %	Lines
src/observables.jl	93.33%	1 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #330   +/-   ##
=======================================
  Coverage   92.43%   92.44%           
=======================================
  Files          41       41           
  Lines        7075     7077    +2     
=======================================
+ Hits         6540     6542    +2     
  Misses        535      535           

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