Add instrument pixel scale property

Closes #31 (closed)

For each instrument it should be possible to compute the size of the pixel scale on the ground.

• Add documentation example on a map with trajectory and projected FOV with a single colorbar.
• Add documentation example of SpiceInstrument properties dump.

Notes:

• not all the instrument provide a IFOV parameter in their instrument kernels
• the pixel scale need to be corrected on the ground when its not looking nadir
• the pixel scale should return a NaN at the limb
• display on the surface (with plot or add_collection) should provide the correct legend and colorbar

Known limitations:

• The POLYGON field of view, we return a NaN value for the iFOV.

List of ik with issues:

• juice_janus_v06.ti
• juice_majis_v05.ti
• juice_gala_v03.ti
• juice_uvs_v03.ti
• juice_pep_v10.ti
• juice_rpwi_v02.ti (code error for JUICE_RPWI_RWI)