absorptionCoefficient_Generic: allow initialization of Xsect with other datatype than float

[Leberwurscht]

The profile=... argument of absorptionCoefficient_Generic in principle allows to supply custom line profiles. We would like to use that for complex-valued profiles, so that the phase upon transmission through a medium can also be calculated - however, this won't work currently because of the line

Xsect = zeros(number_of_points)

where Xsect is initialized as real-valued type. An easy fix would be to allow to initialize Xsect with an optional keyword argument, like suggested in this pull request.

This would allow the user to initialize Xsect manually and thus permit several additional use cases:

• returning complex crosssections
• in-place addition of a new molecule to an existing crossection spectrum
• initialize Xsect with a Python class that fakes to be a numpy array, but records the data of each individual line separately (which we also plan to use so that we can directly synthesize impulse responses in the time domain)