CartesianGrid2D not having magnitude attribute instantiation

[pabloitu]

The class, representing a region, do not have a explicit self.magnitude attribute. However, this attribute is called in some occasions. For example, in

pycsep/csep/core/catalogs.py

Line 717 in 5f84ea9

mag_bins = self.region.magnitudes

In predetermined regions, the attributed is hard-coded into the CartesianGrid2D class.

pycsep/csep/core/regions.py

Line 58 in 5f84ea9

relm_region.magnitudes = magnitudes

which also should be part of the workflow of the modeler. However, CartesianGrid2D.magnitudes should be declared at the class init, and have the possibility to be declared when instantiating.