[Feat] temporal: allow appending to existing STDS

[ninsbl]

Space Time Data Sets (STDS) are type of datas that regularly needs to be updated / extended, when new data comes in.

So far, temporal modules do allow to generate new STDS, but not to append to pre-existing STDS (generated in an earlier run of the module). So, in order to extend / update an existing STDS with a repeated run of a module, users would have to create new STDS and then register the resulting raster or vector maps into the earlier generated STDS.

It would be much more convenient for users if temporal modules that create STDS whould be able to extend / update pre-existing STDS.

Describe the solution you'd like
Temporal modules would have to be adjusted to:

1. Allow output STDS of temporal modules to exists (those that have an output option, not an expression)
2. Possibly add a flag for appending to an existing STDS (and require "ovwerwrite")
3. Possibly ignore meta information (title, description) if STDS exists (if the module provides those options
4. Check that the module run is "compatible" with the STDS to append to (granularity, method, expression, ...)
5. Update the STDS history accordingly (do not overwrite it)
6. Maybe add a standard parser option that accepts both old and new STDS input

The following core modules would likely be relevant:

• t.rast.aggregate
• t.rast.algebra
• t.rast.mapcalc
• t.rast.aggregate.ds
• t.rast.accdetect
• t.rast.accumulate
• t.rast.contour
• t.rast.extract
• t.rast.import
• t.rast.neighbors (see: t.rast.neighbors: support all r.neighbors features and allow to append to existing STRDS as well as filtering by region #3798)
• t.rast.to.rast3
• t.rast.to.vect
• t.rast3d.algebra
• t.rast3d.extract
• t.rast3d.mapcalc
• t.select
• t.vect.algebra

Modules like t.rast.gapfill, t.snap, or t.shift may already contain relevant code as they update / modify existing STDS...