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.