Implement some functions that allow to easily access the instrument database

[ziotom78]

The basic structure of the instrument database has already been defined and described in the documentation. However, there should be some Python functions that allow to easily extract information without the need to do many nested accesses to the dictionary containing the database, e.g.:

print(params['horn']['I0']['ellipticity']

should become something like

print(get_ellipticity(params, beam='I0')