snglcoinc.py:  fix a namespace bug

- glue.segments isn't imported by snglcoinc, need to get segmentlist from segmentsUtils
- edit the docstring for CoincSynthesizer while we're at it.  it had failed to mention that a segmentlist isn't optional if eventlists have been provided.
CoincSynthesizer:  improve "anchor" selection in rate estimation

- measure the volume of the coincidence space and pick the instrument with the smallest
CoincSynthesizer:  fix bias in rate estimation

- this patch removes the (previously documented) bias in the coincidence
  event rate estimation but slows the code down quite a bit
- try speeding the code up a bit, while we're at it, by moving symbol
  look-ups out of the loop.
- overall, it maybe takes twice as long to run in typical cases.