gstlal_compute_strain:  Now allows the updating of line frequencies in the general line subtraction code.

- use `Requires.private` in pkgconfig files to stop forcing downstream
  packages to link against everything upstream
- added `pkgconfig` as `BuildRequires` for RPMs
- bumped required version of pkg-config to 0.18.0

gstlal_compute_strain:  Use short linear phase filter after actuation filters to apply kappas instead of replacing entire static filter.  This preserves the quality of the high-pass filter.

calibration_parts.py:  no longer need to remove initial samples in line subtraction function, since lal_matrixsolver solves that problem.