Detect missing time steps on input data.

We need the ability to detect any missing time steps on the input data.

This will require the following:

- [ ] Determine the frequency and number of input time steps
- [ ] Create an index of the expected time dimension
- [ ] Compare this with loaded time dimension, finding any differences

As always, this will need to be a configurable option in processing to accommodate user needs.

No files or tests are named. Start by locating the input-data processing entry point and its existing configuration options, then trace how the time dimension is loaded. Done means configurable processing can determine the expected time steps, compare them with the loaded dimension, and report missing steps.

Written by the indexing model from the issue text.