Add covariate expression to a function string
addCovariate(funstring, varName, covariate, theta, isLog)a string giving the expression that needs to be modified
the variable to which the given string corresponds to in the model expression
the covariate expression that needs to be added (at the appropriate place)
a list of names of the 'theta' parameters in the 'fit' object
a boolean signifying the presence of log-transformation in the funstring
returns the modified string with the covariate added to function string