Make dask configurable inside pixel_tides

[robbibt]

Proposed changes

Currently the pixel_tides function uses Dask under the hood to resample our low resolution tide modelling outputs into the original higher-res pixels of our satellite data. Previously, we used .compute() inside the function to load our results into memory and return then as a normal xr.DataArray.

However, for certain applications it can be useful to return our tide modelling data as an un-computed Dask array, for example to use as part of a larger Dask workflow.

This PR adds two new params to pixel_tides which make Dask more configurable:

• dask_compute: Defaults to True; if set to False then data will be returned as a lazily loaded Dask array
• dask_chunks: Defaults to "auto"; this will automatically set Dask chunks to those in the input ds dataset (if they exist). Can also be used to pass in a tuple of custom chunks (e.g. (2048, 2048)) to use for the resampling step.

Also added a new test covering this functionality, and re-run the How_to_guides/Tidal_modelling.ipynb notebook.

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[vnewey]

Thanks Robbi!