point selection function

[rogerkuou]

In classification.py implement the ps_selection function with two flavors: NAD and NMAD.

• Move NAD function from sarxarray implementation to here.
• Add NMAD implementation by Wiestke, which is attached below.
• Add deprection warning to point_selection function in sarxarray

def _nmad(self, chunk_azimuth=500, chunk_range=500):
        """
        Estimate the NMAD for a xarray dataset. 
        """
        # Time dimension order
        t_order = list(self._obj.dims.keys()).index("time")
        
        # Rechunk to make temporal operation more efficient
        amplitude = self._obj.amplitude.chunk(
            {"azimuth": chunk_azimuth, "range": chunk_range, "time": -1}
        )

        median_amplitude = amplitude.median(axis=t_order)
        mad = (np.abs(amplitude - median_amplitude)).median(dim="time")
        nmad = mad / (median_amplitude + np.finfo(amplitude.dtype).eps)

        return nmad