Improve naming consistency of methods which convert between UxDataArray, UxDataset, xr.DataArray, and xr.Dataset

[Sevans711]

EDIT: the issue title was focused on proposing to implement UxDataArray.to_uxdataset() and UxDataset.to_uxdataarray(), but I later discovered the relevant functionality actually exists, it just wasn't named what I was expecting. See next message for details.

Below was the original text for this message, less relevant now but I am keeping it here for reference:

These should both work in the intuitive way, analogously to xarray.DataArray.to_dataset() and xarray.Dataset.to_dataarray().

These may be especially relevant as an easy-to-implement workaround for #1734 (and could maybe even be used while solving that issue?), as it would enable something like ds.to_uxdataarray('variable').zonal_mean().to_dataset('variable'), and similar for any other currently-only-defined-for-UxDataArray methods.

[Sevans711]

Ah! Woops… both of these actually already exist, they just weren't named what I was expecting! The relevant functions are UxDataset.to_array() and UxDataArray.to_dataset().

I had previously tried UxDataset.to_dataarray() (which returns an xarray.DataArray) and UxDataset.to_uxdataarray() (which doesn't exist) and then (incorrectly) assumed that meant there wasn't any method that would work the way I had hoped.

Looking into it further, here are all relevant functions for converting between UxDataset, UxDataArray, xr.Dataset, and xr.DataArray:

From \ To	UxDataset	UxDataArray	xr.Dataset	xr.DataArray
UxDataset	---	to_array()	to_xarray()	to_dataarray()
UxDataArray	to_dataset()	---	Not Implemented	to_xarray()
xr.Dataset	(constructor)	(constructor)	---	to_array() or to_dataarray()
xr.DataArray	(constructor)	(constructor)	to_dataset()	---

where "(constructor)" implies to use the appropriate constructor syntax, along with supplying a uxgrid, to build the uxarray object in the first place, such as uxarr = ux.UxDataArray(ds.to_array(), uxgrid) to go from xr.Dataset to UxDataArray.

This table feels a bit inconsistent / unintuitive to me, for a few reasons:

1. Why provide something for UxDataset → xr.DataArray but not for UxDataArray → xr.Dataset?
2. Why should to_array() and to_dataarray() be different functions for UxDataset but aliases of each other in xr.Dataset?
3. Why do the method names not include ux (as suggested in the title of this issue report), when other variables do (e.g., it's obj.uxgrid, not just obj.grid)?

I'm renaming and relabeling this issue appropriately and moving it back into its "untriaged" state now that it is clear the relevant functionality does actually exist, the naming just wasn't intuitive for me.