Usecase/molecular atmosphere

A first merge request with the usecases addressing the computation of the Molecular Extinction Profiles (MEP). Concerning the Refractive Index Profile (RIP) usecase, it follows the same path as the MolecularExtinctionProfile.md, so it was not added for the moment.

This MR modifies also the main atmospheric calibration usecase according to the issues opened by Markus:

Closes #46 (closed)

Closes #47 (closed)

Closes #48 (closed)

Closes #49 (closed)

Closes #50 (closed)