Instrument pixel scale

Currently there is not an easy way to compute the the pixel scale for a given instrument.

For MAJIS it could be done as:

from moon_coverage.ticks import m_pix_ticks

majis_m_pix = traj.observer['IFOV'][0] * m_pix_ticks * 1e3

But for JANUS it will be:

janus_m_pix = traj.observer['IFOV'] * m_pix_ticks * 1e3

Because, IFOV return the reference and the cross-track angles, in the second, one 1 value is return.

For other instrument, typically GALA_RXT, the IFOV is not defined in the instrument kernels so the user could use the FOV_REF_ANGLE and FOV_ANGLE_UNITS.

We need a more standardized way to retrieve this information.