Add occuCOP: occupancy model with count data

[leapautrel]

I mostly added:

• unmarkedFrameCOP the child class of unmarkedFrame to organise count data for occuCOP
• occuCOP the function to fit the model, that calls a getDesign method, likelihood functions (in R and in C++)
• unmarkedFitCOP the child class of unmarkedFit to organise occuCOP outputs
• Tests and documentation for occuCOP
• A guide to add a new model to unmarked as a vignette with the steps I took