added the _rebin function
implement lacosmic.   Added _blkavg as well
pep8 fixes to cosmicray tasks
replaces a bracket accidently deleted
add blkavg tests
Added additional tests to test_cosmicray.py
Added parameters to call of cr_func
removed parameters from cr_func and added background test
Merge pull request #124 from crawfordsm/lacosmic

Lacosmic