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