Support for constraining future scenarios with zones dynamically

[Martin-Jung]

One effective way to constrain projections is through zoning layers or mcp's. At the moment these can be added either via add_control_extrapolation() or directly via the limits parameter in distribution() or scenario(). Particular for the latter it would be good if scenarios could be dynamically constrained, for example by an expanding climate zone.

Propose to add this option to the package:

• Add support for a list of stars for zones to add_control_extrapolation(). In case no projection is made, the first entry at point t is used
• Check that similar functions work for distribution() or scenario().
• Small unit tests to check that this works and does not cause issues downstream.

Also further:

• Convert entire zone handling to terra owing to bugs in factor conversion