- use gpsproperty() from lsctables instead of implementing it from scratch

gstlal_compute_strain:  Commented out 2 lines as a temporary fix to get SRC detuning calculation to work.

calibration_parts.py:  Added new function remove_lines.

- use fewer intermediate variables when parsing caches
- refs #5589

- this patch removes use of the glue.lal.Cache class
- refs #5589

- fall-through is intended, but new gcc 7 complains about it

- found by new implicit fall-through warning in gcc 7

- the occurrence of an error in the start() method was not being reported to the calling code

- package dependencies change following reorganization of GDS packages
- now require only gds-lowlatency package

- gets doctest working

Some debugging on gstlal_clean_strain (although it still doesn't run)