Expose parameter to control reference catalog density for crowded field astrometry

[drhaz]

See issue report
https://groups.google.com/forum/#!topic/odi-ppa-help/G-19epHTBTE

Whereas a quick fix was done again on the source code level by the operator, it would be helpful to expose to density rejection parameter to the command line of collectcells in order to tweak the pipeline execution in PPA.

See tweaks in branch testing_crowdedfield_2.3

[rkotulla]

addressed with fixes in branch crowding_flag. Use -crowded on command line to switch WCS catalog to use primarily bright and isolated stars.
Ready for testing!