Ability to slice on an FC Dataset

[kkappler]

Context:

If a run is very long, and we have already stored the FCs for the whole run, it would be nice to be able to slice a sub-spectrogram out of the mth5.

I'm not sure where this would go -- the functionality would be similar to the function RunGroup's to_run_ts() method in mth5/mth5/groups/run.py, which takes optional start, end keyword arguments.

but it could perhaps go into to_xarray() method in mth5/mth5/groups/fourier_coefficients.py

In this case, because the FC Dataset is 2-dimensional (time and frequency) ideally we could slice on either of these dimensions, so the kwargs could be start, end, frequency_min, frequency_max for example