Output files Time is not a Dimension Coordinate

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

The time coordinate isn't always a dimension coord in LFRic. Generally it is not

the I/O system tends to define a dimension called `time_counter` but a coordinate of `time_instant`
```CF
double time_instant(time_counter) ;
```

This is valid CF, but there are semantic difference between this and a dimension coordinate.
This issue shall explore potential for change, effort and impact. This is not infinitely configurable with XIOS.

### Further details of the issue.

_No response_

Start by examining how LFRic output files define the time_counter dimension and time_instant coordinate, then review the relevant XIOS configuration and CF semantics. Determine whether changing the representation is feasible, what output and compatibility impacts it has, and record a clear recommendation or implementation scope.

Written by the indexing model from the issue text.