v2.10.0

Transmissive materials with tabulated optical constants and measured transmissivity (#240)

Two new materials let a coated window be modeled from a measured
transmission curve while still refracting and absorbing like the bulk
material it is made of.

• optika.materials.Dielectric takes its complex index of refraction from
  the tabulated optical constants of a chemical (Dielectric("MgF2")),
  rather than a Sellmeier fit like Glass. It refracts through Snell's law
  with the tabulated real index and attenuates transmitted rays with the
  imaginary part, so it works in the ultraviolet where the Sellmeier fits of
  common glasses are not valid.
• optika.materials.MeasuredFilter is the transmissive counterpart of
  MeasuredMirror. A measured transmissivity is applied once at the
  interface, while refraction and absorption are delegated to a medium.
  A coated window is a pair of surfaces: a front surface made of
  MeasuredFilter(medium=Dielectric("MgF2")) and a back surface made of
  Vacuum. Since a coating can only be measured through its substrate,
  is_medium_measured defaults to True and ignores the medium's
  attenuation so it is not counted twice; set it to False for a
  measurement reduced to the interface alone.

MeasuredMirror is unchanged in behavior; its wavelength interpolation is
now shared with MeasuredFilter.