Deprecate datasets module

[santisoler]

Description of the desired feature

As decided on one of the last Community Calls, every Fatiando dataset will be available for download through Rockhound.
After fatiando/rockhound#84 is merged, we should add deprecation warnings for the verde.datasets module and deprecate it for Verde v.2.0.0. The module should be removed along with the data folder. Besides, all gallery examples should download their data from Rockhound, therefore it should be added as an optional dependency.

Are you willing to help implement and maintain this feature? Yes, but would leave to anyone that wants to tackle it.