Add support for KS3 spectroscopy diagnostic

[vsnever]

The data from KS3 spectroscopy is used for benchmarking the EDGE2D-Eirene, SOLPS and ERO 2.0 simulations. Having the KS3 lines of sight in the Cherab module for JET would allow, for example, to estimate the levels of reflected light in the measured signals.

For KS3 sight lines, it is important to consider the non-zero acceptance angle of the observation cones, so they should be modelled with FibreOptic and not SightLine. Hence, a possible solution would be to update SpectroscopicSightLine in cherab.tools to support both SightLine and FibreOptic observers.