No overlapping times detected in input cubes when plotting time series

### Describe the bug

When plotting time series you can get the following error:

ValueError: No overlapping times detected in input cubes.

This comes from a check in collapse.py that we have overlapping times. While this check makes sense for a difference plot, it does not make sense for other kinds of plots, such as a time series, where it is quite reasonable to have non-overlapping times.

https://github.com/MetOffice/CSET/blob/01277f21c9261e06863d48c56aaf2e72a05bc9b2/src/CSET/operators/collapse.py#L77-L89

We should move this logic so it only applies in cases where we only want overlapping time points, such as the difference plots.

### How to reproduce

Steps to reproduce the behaviour:

1. Run CSET with two models over different dates.
2. Try and plot a time series.
3. Observe the error.

### Expected behaviour

Time series plots should still work when we have non-overlapping data.

Start by reading the overlapping-time check in src/CSET/operators/collapse.py at lines 77–89, then reproduce the issue by plotting a time series from two models with different date ranges. The change is done when non-overlapping inputs work for time-series plots while the check remains for plots that require shared time points.

Written by the indexing model from the issue text.