covariates for marine envs

[timcdlucas]

Bioblim module currently crops to terrestrial land.

There should be either a marineBioclim module or a switch in Bioclim to switch this.

Both terrestrial and marine is potentially an appropriate extent as well.

[timcdlucas]

Sorry, bioclim only returns terrestrial data I think. We should add a marine data module.

Perhaps http://www.oracle.ugent.be/download.html