hakcing, modifying function to be useful with plyr

not working, committing while I change places
seems to work on I5, N

now to run it with plyr invoking
refactor, pull function into its own file
set up for parallel execution

also pass in wim df as an extra parameter to grouping function

modified:   R/groupsites.R
modified:   R/process.vds.files.R
testing

tests are failing the ability to add priority site
test for using VDS as priority sites passes

also noticed that "limit" was hard coded into the groupsites code.
switched it for distance.

also noticed that I'm not really checking to see whether or not a
detector is farther away from others right now.  That is, I am using
"distance" in assigning the priority sites, but if two priority sites
are less than "distance" apart, then one of them might not be used.
factored out groupsitesr to its own package
fixed to work with groupsitesr

and the plyr-ing of groupsites works okay

can process all of vds sites, with WIM sites, and combine by freeway,
direction

haven't done the plotting yet
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