Investigate allowing the construction of `UxDataset` from a single grid & data file

[erogluorhan]

While most unstructured grids datasets are broken down into separate grid and data files, there are cases where both could be stored under a single file.

Having an option to parse a single dataset for both grid and data variables would allow for support of these types of files.

Originally posted by @philipc2 in https://github.com/UXARRAY/uxarray/pull/216#discussion_r1226969655