Reexport assignment functions for obs, media and deployments

[damianooldoni]

Reexporting camtrapdp assignment functions:

• deployments<-()
• observations<-()
• media<-()

This is a feature users need to do some basic datapackage manipulation, for example to correct some errors in the deployments resource as exported by the camera trap management application (e.g. Agouti), or to check some modelling behavior by creating synthetic data starting from an existing dataset.

[damianooldoni]

Close as completed via 520197d.