More relaxed check between ref and model time coordinates (fixes #92)

Still running a few tests, so marking as draft.

I've changed the strict equality requirement of the time coords between the ref and model, to the following checks:

1. Number of time points must be the same (i.e. same array shape)
2. Time coord points must be equal (within a small relative tolerance)
3. Time bound intervals must be of the same length (within a small relative tolerance)

I've rearranged the suggested implementation a bit, to separate the error messages of the potential errors.