Add Functionality to convert gdf and point data to xarray

[ValentinGebhart]

Is your feature request related to a problem? Please describe.
There are regular requests about writing intensity maps / exceedance intensity maps / return period maps to xarray raster data (and then netcdf files), e.g., #854, #857, #1013. The functionality is essentially already there, see util.plot.geo_im_from_array()

Describe the solution you'd like
We could separate the rasterize functionality from util.plot.geo_im_from_array() to a new function, if the interpolation used there (scipy.interpolate.griddata) is what we think makes sense. We could also make rasterizing optional, i.e. one could simply transform the point data to a 1d xarray if one wants.

def xarray_from_gdf(gdf, rasterize=True):

or more generally

def xarray_from_points(values, lon, lat, rasterize=True):

[peanutfun]

one could simply transform the point data to a 1d xarray if one wants.

This appears rather trivial to me. Why would we need that?

We could separate the rasterize functionality from util.plot.geo_im_from_array() to a new function, if the interpolation used there (scipy.interpolate.griddata) is what we think makes sense.

The geo_im_from_array() tries to guess if the grid is regular. If it is, it reshapes the data into a suitable 2D array. If not, it interpolates onto a fixed resolution regular grid. I think this is what we want and what the user should be able to control:

def to_grid(
    gdf: geopandas.GeoDataFrame,
    interpolate: Optional[bool] = None,
    resolution: int = 250
) -> xr.DataArray:
    regular, _, _ = grid_is_regular(gdf.geometry)

    if not regular:   # We must interpolate
        if interpolate is None:  # User does not care
            logger.warning("Interpolating non-regular grid")
            interpolate = True
        elif interpolate is False:   # User does not want interpolation
            raise RuntimeError("Cannot reshape non-regular grid")

    if interpolate:
        data, coords = interpolate(gdf, resolution)
    else:
        data, coords = reshape(gdf)

    return to_xarray(data, coords)