allCombinations, AICc and logLik method, tweaks

* Tweaked occSScovar and occSScovarSite to improve convergence and catch
variance estimation problems; covars now internally standardised.
* Added allCombinations function.
* Added logLik and nobs methods for class occupancy; modified occSS*
functions to accommodate that. Function stats::AIC now works for
occupancy.
* Added AICc generic and AICc.default.