[Feat] t.rast.univar: add option(s) for temporal aggregation

[ninsbl]

Is your feature request related to a problem? Please describe.

r.univar accepts multiple input maps. It seems that this feature could be used in t.rast.univar to compute univariate statitics over one ore more maps in a temporal granule, meaning to compute temporal aggregated statistics.

Describe the solution you'd like
In addition to listing single maps, map lists could be generated grouped by a user defined granularity maybe with a user defined start point, using two new options for the module, like e.g.

granularity="24 hours"
start="2001-01-01 06:00:00"

Describe alternatives you've considered

Currently, one would run t.rast.aggregate and then t.rast.univar. The drawback there is, that new data is generated that occupies storage. For data where one frequently computes statistics that could make sense any way.