Adding in tests for gstlal-calibraiton-1.2.1 release, fixing a few of the test scripts, and updating gstlal_compute_strain to use pipeparts mkchannelmux function

- tests/check_calibration:  Caitlin Rose's Automated testing scripts and Makefiles for doing calibration tests.
- gstlal_compute_strain:  Don't demodulate the pcal channel again to compute SRC detuning if re-using the ~30 Hz pcal line
- lal_transferfunction:  1. Bug fixes for median of transfer functions.  2. Option to compute first transfer function asap at start of stream.  3. Use GPS time to decide when to update transfer functions so that output becomes independent of start time asap.
- tests/check_calibration:  Added plotting script and Makefile example for plotting time series of \DeltaL / pcal at calibration line frequencies.
- config_files:  Example config file for offline calibration with noise subtraction.