Add spectroscopic instruments

[vsnever]

This issue is suggested by discussion in cherab/jet#21.

Classes for spectroscopic instruments such as polychromators, survey and high-resolution spectrometers, and others can simplify the setup of pipelines and spectral properties of observers. Therefore, it makes sense to add base classes for such instruments in the tools module of the core package.

[jacklovell]

Fixed by #300.