introduce Mirror struct for mirror thermal calculations

This struct is created by the precomp_mirror function, and is essentially the union of the Material and Optic structs, holding all relevant properties for a given optic for thermal noise calculations. It can be used in place of either the Material or Optic structs.

Coating thermal noise functions are modified to accept this struct.

mentioned in merge request !95 (merged)

approved this merge request

merged

mentioned in commit 12d568be