Fix C-matrix definition and instrument attitude

Based on SPICE documentation, C-matrix (camera matrix) transforms the coordinates of a point in a reference frame (like J2000) into the instrument fixed coordinates:

[ x_inst]     [          ] [ x_J2000 ]
| y_inst|  =  | C-matrix | | y_J2000 |
[ z_inst]     [          ] [ z_J2000 ]

The transpose of a C-matrix rotates vectors from the instrument-fixed frame to the base frame:

[ x_J2000]     [          ]T [ x_inst ]
| y_J2000|  =  | C-matrix |  | y_inst |
[ z_J2000]     [          ]  [ z_inst ]

Currently, the planetary_coverage.spice.toolbox.attitude() is computing the transpose of the C-matrix and not the C-matrix:

sp.pxform(str(observer.frame), ref, time)

whereas it should be:

sp.pxform(ref, str(observer.frame), time)

A better docstring should describe this matrix with the example above.

Currently, the trajectory objects provide a sc_attitude property base on planetary_coverage.spice.toolbox.attitude(). It should be updated to take the transpose of the results.

Additionally, this sc_attitude is computed for the observer (spacecraft or instrument) frame. This is not consistent with the sc_ prefix. We recommend to depreciate this property (with a warning) and provide a generic attitude property that takes into account the nature of the observer. This will also affect the radec values that are also observer specific.

/cc @lpenasa