foucalc: Add rms qualifier with error calculation

- The "RMS" qualifier calculates the RMS of the lightcurves
  (given by rate1 and rate2), and returns it as an array, however,
  without error.
- A new qualifier "rms" is added, which calculates the RMS of the
  signal PSD (same as "RMS" qualifier), noise PSD and the error
  according to Uttley et al., A&A Review 22, 72, 2014, Sect. 2.2.3.
- The reference variable is henceforth of Struct_Type
- Backwards-compatibility: The "RMS" qualifier is labelled
  "deprecated" and a warning is is thrown
- This commit also remove some old commented-out code