Fix transform of CameraGeometry

[kosack]

Allow a Frame class to be passed so attributes are retained

Fixes #2277

The following now works:

from ctapipe.instrument import SubarrayDescription
from ctapipe.coordinates import CameraFrame, EngineeringCameraFrame
import astropy.coordinates as c

sub = SubarrayDescription.read("dataset://gamma_prod5.simtel.zst")
geom= sub.tel[100].camera.geometry
newgeom = geom.transform_to(EngineeringCameraFrame)
print(geom.frame)
print(newgeom.frame)

<CameraFrame Frame (focal_length=16.445049285888672 m, rotation=0.0 rad, telescope_pointing=None, obstime=None, location=None)>
<EngineeringCameraFrame Coordinate (focal_length=16.445049285888672 m, rotation=0.0 rad, telescope_pointing=None, obstime=None, location=None, n_mirrors=1): (x, y) in m
    [( 0.   ,  0.   ), ( 0.   , -0.049), (-0.049,  0.   )]>

Note that the focal length and other parameters are retained in the transform

[kosack]

TelescopeFrame will still lose the info, but I guess that is fundamentally because it doesn't contain that attribute.

sub = SubarrayDescription.read("dataset://gamma_prod5.simtel.zst")
geom = sub.tel[100].camera.geometry
print(geom.frame)
print(geom.transform_to(EngineeringCameraFrame).frame)
print(geom.transform_to(TelescopeFrame).frame)

<CameraFrame Frame (focal_length=16.445049285888672 m, rotation=0.0 rad, telescope_pointing=None, obstime=None, location=None)>
<EngineeringCameraFrame Coordinate (focal_length=16.445049285888672 m, rotation=0.0 rad, telescope_pointing=None, obstime=None, location=None, n_mirrors=1): (x, y) in m
    [( 0.   ,  0.   ), ( 0.   , -0.049), (-0.049,  0.   )]>
<TelescopeFrame Coordinate (telescope_pointing=None, obstime=None, location=None): (fov_lon, fov_lat) in deg
    [(0.        , 0.        ), (0.        , 0.17071965),
     (0.17071965, 0.        )]>

However, if the original CameraFrame has an obstime or telescope_pointing, they is retained, as they are attributes of TelescopeFrame as well. I wonder if we should add focal_length to TelecopeFrame, so full round-trips are possible?

[kosack]

from the Astropy docs:

The same lower-level frame classes also have a transform_to() method that works the same as above, but they do not support attribute-style access. They are also subtly different in that they only use frame attributes present in the initial or final frame, while SkyCoord objects use any frame attributes they have for all transformation steps. So SkyCoord can always transform from one frame to another and back again without change, while low-level classes may lose information and hence often do not round-trip.

[kosack]

as mentioned in the issue, I think I will modify this to store the pix_x, pix_y coordines internally in a SkyCoord, rather than constructing a SkyCoord during transform. That will allow all frame attributes to be propegated, even ones not explicitly in the target frame. That means one will be able to transform from CameraFrame to TelescopeFrame and back again without re-defining the parameters.