STIX Imaging frame

[samaloney]

Have implement the frame but need to:

• [x] check the transformations
• [x] add mapping between WCS and frames so plays nicely with Map
• [x] test reprojection to and from

[paolomassa]

Is it already implemented a function for reading the aspect solution from solo_L2_stix-aux-ephemeris fits files? See stx_create_auxiliary_data.pro