Ntv-numpy

- add Xarray accessor (xr.Dataset.nxr.*)
- pre-commit integration

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- bug 3 (python 3.10)
- enh 4 (Unification of parameters in to_xarray, to_scipp and to_dataframe)

- add NTVtype extension (Ndtype class)
- add 'uniques' xndarray
- interface:
- pandas.DataFrame

- First version
- exchanging via JSON format
- lightweight format (structure: json, arrays: URI)
- numpy.dtype supported: all except 'complex' and 'timedelta'
- NTVtype supported:
- datetime, date, time, dat
- month, year, day, wday, yday, week, hour, minute, second
- json, string, number, boolean, array, object, null
- floatxx, uintxx, intxx
- base16, decimal
- uri, ipv4, email, file
- point, line, polygon, geometry, geojson
- multipoint, multiline, multipolygon, box, codeolc
- ndarray, xndarray, field, tab, ntv
- interface:
- astropy.NDData
- scipp.Dataset
- Xarray.Dataset, Xarray.Dataarray
- JSON