provide functions for coordinate systems and transformations

[DavidWalz]

It would be beneficial if a common naming would logically group the functions together, as it is currently done with SourceXXX and ObserverXXX.

e.g.

from crpropa import *
v1 = coordinates_galactic(124, -32, degree=True)  # 3-vector in galactic coordinates, |v1| = 1
v2 = coordinates_gal2sgal(v1)  # the same vector in supergalactic coordinates
v2 = coordinates_gal2eq(v1)  # the same vector in equatorial coordinates
...

[DavidWalz]

We discussed about this last year based on Daniel's suggestion.
However this feature is provided by many external libraries and I don't see it a necessary part of CRPropa.