Synchronize: overwrite if local file is older than cloud

[visr]

#482 made things less fragile, but also more frustrating in some ways

• data downloads every time, download is slow
• if you are working on updating data locally, this will overwrite your local changes without warning, leading to data loss

We could address these by, by default, overwrite only if local file is older than cloud. We'd need to be able to cheaply check online timestamps for this. And this introduces the risk that after producing new input data, you forget to upload it. This could be mitigated by writing our own pre-commit hook.

Although I wonder if it is not just better and less work to just start moving this into DVC directly. That should have similar behavior as we want.

When we have this a a default we should directly remove riskier file existence checks such as https://github.com/Deltares/Ribasim-NL/pull/490/changes#diff-95be113a59d0159f00482af4b13d5ed4baa5c4822ddc55147069d2aab967be84