Model data types with classes

[Jaapel]

If you read the documentation of HydroMT, different data types are explained as a clear entity in HydroMT.
These data types are not represented as a different entity in the code:
A rasterdataset, dataset and geodataset are all represented by a xr.Dataset with different properties (and the vector and raster accessors defined in the gis submodule. A dataframe is a pd.DataFrame and a geodataframe is a gpd.GeoDataFrame.
This makes the signatures of different functions hard to use. Different methods should live close to the data are now part of different classes in HydroMT. Examples include:
mosaicking in a rasterdataset, get_bbox, get_timerange and all different utility methods now spread throughout the code.

I propose to have different data classes that we use in the code. I know that the rasterdataset is not implemented as an xarray accessor: https://docs.xarray.dev/en/stable/internals/extending-xarray.html, but a thin wrapper that gives us a clear what methods there are and which arguments are valid will help keep the code maintainable.