rename_labels in time dimension

[jdries]

Example use case provided by CloudInSAR:
cloudinsar/s1-workflows#56

To perform operations between dates, it can be necessary to change the temporal key assigned to a tile.

Currently, this is simply not supported. I also believe we will require source and target dates to be set, to correctly perform a mapping.
If source dates are not set by the user, we can off course do an expensive collect operation on the RDD, or get them from the partitioner in certain cases.

Note that there are also workarounds for this issue via UDF.