burca_tailor:  consolidate CoincParamsDistributions

move binnings, filters and coinc_params funcs into CoincParamsDistributions classes
burca_tailor, power_plot_burca2;  port to ligolw_burca_tailor.load_likelihood_data
pylal:  move CoincParamsDistributions class from ligolw_burca_tailor to snglcoinc

also move some associated I/O code along with it
move LikelihoodRatio class from burca2 to snglcoinc
snglcoinc:  move PDF interpolator construction

from LikelihoodRatio class to CoincParamsDistributions class' .finish() method
stringutils:  move .add_slidelessbackground() to CoincPramamDistributions
snglcoinc.CoincParamsDistributions:  convert .copy() to regular method
snglcoinc:  move .P() method from LikelihoodRatio to CoincParamsDistributions

and replace .P() with .P_noise() + .P_signal()
also add .lnP_noise() and .lnP_signal()
snglcoinc.CoincParamsDistributions:  track counts and PDFs separately

add separate BinnedArray attributes containing the PDF data, and have the
.finish() method populate them and leave the raw counts alone.  also record
the PDFs in XML serializations so that .finish() doesn't necessarily need
to be called.

these features have been added in an ad hoc way in several applications.
for example the gstlal-based inspiral pipeline implements much of this in
the DistributionStats class, and the excess power and string cusp pipelines
provide the option of working with likelihood data in "counts" or "pdf"
form via command line options (leaving it up to the user to remember which
of the two they have in their .xml files).  implementing this here will
allow clean ups in those codes.  by preserving both the bin counts and the
PDFs, finally the .__iadd__() method knows how to weight the PDFs when
combining them.
snglcoinc:  work on CoincParamsFilterThread

- switch default implementation to filter_array() + .to_pdf()
- add filterthread keyword argument to .finish() method to allow override
- allow filters to be optional
- use ProgressBar for verbosity
pylal:  clean up lalapps_burca_tailor
pylal:  clean up lalapps_string_meas_likelihood
snglcoinc.CoincParamsDistributions:  incorporate process_id into class

and don't pass it separately through the I/O routines
snglcoinc:

- clean up ligolw import namespace
- introduce CoincParamsDistributions.get_xml_root() method to facilitate overriding .from_xml()
move stringutils' instrument categories stuff to snglcoinc
snglcoinc:  use << instead of pow() for powers of 2