danielrd6/ifscube: Modeling

This release has major improvements on the modeling of spectral features, with a severe revision of the code. Some issues still remain, and there are some left overs in a few classes. The full transition towards this new structure is planned for version 1.2.

Key points:

• Multiple component fitting is based on replicating kinematic parameters, instead of adding constraints to the minimizer.
• It is possible to choose the minimization method in the configuration file. Besides the methods available via the scipy.optimize.minimize wrapper, there is also the possibility of using differential_evolution. Constraints are changed appropriately, and the conversion is performed internally.