Change function name negbin to negbin_ll for consistency.  Add MLE solver for parameters in truncated negative binomial distribution.
Change function name negbin to negbin_ll for consistency. 
Add MLE solver for parameters in truncated negative binomial distribution.

davharris/macroecotools,weecology/macroecotools,rueuntal/macroecotools,weecology/macroecotools,rueuntal/macroecotools