Adding an example Makefile for running gstlal_compute_strain in O2, a filters file, and an example plotting script for O2

gstlal_compute_strain:  Added calibration line subtraction back in. There's also now a prototype function for subtraction of 60 Hz lines and harmonics, and for noise subtractions using witness channels.

- new package cmath: elements that perform unary operations and handle complex and real floating point arithmetic. Elements included are cabs, cexp, cln, clog, clog10, and cpow.
- new element lal_trackfrequency: base transform element that attempts to measure the loudest frequency in a signal.

- lal_smoothkappas: added filter latency option.
- lal_resample: simpler treatment of gaps. The complicated nonsense from before was unnecessary and messed things up.
- calibration_parts.py: function to remove lines and harmonics with a witness channel now works well.

- lal_complexfirbank:  Now handles latencies longer than filter length.
- lal_insertgap:  Compute offset when throwing away initial data as offset relative to the time when streaming started.
- calibration_parts.py:  remove_lines_with_witness should work now.

-the code needs to know how many input samples are needed to drain enough of the history for there to be nothing but 0s remaining.  to determine the amount of non-zero history, instead of counting contiguous non-zero samples from the start of the history count contiguous zero samples from the end and subtract from the total.

gstlal-calibration:  adjust gstreamer plugin .so filenames on gstreamer 1.14+ to work around changes to name mangling used by the plugin scanner

lal_transferfunction:  fixed off-by-one bookkeeping bug in storing old data, and property-setting bug

lal_transferfunction:  fixed Tukey window bug.  Also use comple conjugate when resampling transfer functions and going past the edges.

lal_transferfunction:  Made fir filter length independent of fft length. Added ability to resample and low-pass filter transfer functions to produce fir filters. Reduces noise in transfer functions wieh multiple witness channels are equally well correlated with the signal to be cleaned.

lal_transferfunction:  allow any length of fir filters by resampling the transfer functions. Reduce noise by low-pass filtering transfer functions.

lal_transferfunction:  Only update transfer functions and FIR filters if they were computed successfully.

audioresample_test.py:  simple pipeline to compute the transfer function associated with downsampling and upsampling data with audioresample

lal_transferfunction:  if transfer function of fir filter generation fails for any reason, immediately try again.

gstlal_compute_strain:  added option to remove DC component from residual and control channels before filtering.