smurf: Replace // C++ style comments with /* C style comments */
smurf:  Document previous ZPD conversion from mechanical to optical units
smurf: Remove reverse scan mirror position inversion

- Attempting alternative implementation to avoid having to reorder corresponding data
- Swap start and end mirror position index values for reverse scan case instead
- It is the caller's responsibility to detect this and adapt accordingly
- Detect reverse scan by way of start and end values from fts2_validatemirrorpositions
  which no longer reverses the scan list (to avoid having to reorder corresponding data)
Revert "smurf: Remove reverse scan mirror position inversion"

- This attempt to handle reverse scan data directly failed
  because the GNU Scientific Library's  gsl_spline_init function requires
  monotonically increasing values

This reverts commit b37f55f8a6afc004d34464610104eefa0c69c02a.
smurf: Reverse data also with mirror position array in case of opposite scan direction

- Previous code only reversed the mirror position array, but not the corresponding interferogram data