Add omegamap option

[pablosanjose]

Closes #242

The solution is to provide support in densitymatrix and josephson (for the moment) so that the integration variable omega gets mapped to ParametricHamiltonian parameters through an omegamap function.

Example: J = josephson(ph, ωmax; omegamap = ω -> (;ω), ...), where ph::ParametricHamiltonian contains a parameter of name ω, introduced e.g. by an @onsite((; ω) -> Σ(ω)).

CC: @CarlosP24