CLN: Moved image percentile code into separate function, for profiling
PERF: Allow duplicate maxima filtering to be turned off
TST: Check proper rejection of local maxima near edge.
BUG: Reject all invalid features near image edge.

See discussion in https://github.com/soft-matter/trackpy/pull/139
CLN: Remove unused separation argument to local_maxima()
Merge pull request #139 from nkeim/separation-fix

BUG: Choose margin based on several factors that disqualify features near the edge.