- this is a rewrite of gstlal_inspiral_lvalert_background_plotter to cause
  it to pull all input data from the gracedb candidates
- the patch makes writing the plots to a local disk optional (and the
  default is not to)
- the patch makes uploading the plots to gracedb optional
- there are several reasons for wanting this rewrite:
  - the plots shown in gracedb are certain to be of exactly the data used
    to rank the event, not whatever is left on disk from the last snapshot
    of the online analysis.
  - the plots don't get successfully generated unless gracedb gets a copy
    of the data required to reproduce the event's rank, providing some
    quick visual indication of candidates that might not have a complete
    record of their origin stored in gracedb.
  - the plots can be generated on a different machine than the one running
    the analysis, a machine that might have a newer (nicer) matplotlib
    installed or might have more free CPU cycles
  - if somebody modifies the program, makes it generate new plots, or
    changes the plots somehow, old events can can have new plots
    (re)generated for them without having to find the machine that was
    running the analysis or the directory where the analysis was run.
  - the tool can be used to generate plots for other searches if they
    provide compatible ranking statistic data (they won't, of course,
    because the ranking statistic data required by this program is too
    tightly coupled to the gstlal-based pipeline, but here it's the thought
    that counts)
- refs #2423