Incorporated model covariance into nestlc function

Added modelcovariance to nested routines. I think two further changes should be done:
1. we should make modelcov=True the default
2. Routines like _chisq should not have a default value of modelcov, and should use the value of modelcov 
specified by the routines like chisq. Alternatively, if this is not convention we want to follow, I should change _nestlc. At least these should be consistent.
I think this should be the `_chisq()` function rather than `chisq()`. They do the same thing, but `_chisq()` is faster. Also, can you remove the commented-out lines entirely?