add generic StacResource.process

[soxofaan]

Users that wanted to use export_workspace before version 0.39 had to do something like this:

saved = cube.save_result(...)
res = saved.process("export_workspace", arguments=...)

Since 0.39.0, this fails because res does not have a process method. Before 0.39, res was a DataCube, which made little sense, but at least it has the generic process method.

Quickfix would be to add a generic process to StacResource too

related to #720 #402 #725