v0.2.0

Changed

• Breaking: the sign convention of r_p. solve_tmm(..., polarization="p").r
  and fresnel_r(..., "p") now return r_p = (n1 cos2 - n2 cos1)/(n1 cos2 + n2 cos1),
  so r_p = r_s at normal incidence (air to glass: -0.2 instead of +0.2) and
  r_p -> +1 at grazing incidence. This is the opposite sign of Hecht / Born &
  Wolf; multiply by -1 to get the old values. t, R, T and A are unchanged.
  THEORY.md describes both conventions and why neither looks symmetric at both
  normal and grazing incidence.

Fixed

• THEORY.md formulas render correctly on GitHub: they use GitHub's protected
  math syntax, so Markdown no longer eats underscores and backslashes.