Make output mesh CLI argument in precice-aste-run optional

[davidscn]

Currently, using precice-aste-run without an --output argument leads to a strange error, i.e., the user is forced to specify an output mesh. If a user (for whatever reason) wants to get the maapped data on the same mesh as used for the input, the user could define the input mesh as an output mesh. However, this will simply override (i.e. erase) the input mesh and all the data it contains.

We should allow the user to omit the --output flag and append the mapped data to the already existing data sets in such a case.