Make UDFMeta.coordinates available in get_results

[sivborg]

Hello, one useful feature that's missing is access to the _slice in the get_results member function in UDFMeta.
One use case I am currently working with is using the coordinates there in order to calculate regression at their positions:

File ~\Prog\LiberTEM\LiberTEM\src\libertem\udf\raw.py:104, in PickCorrectedUDF.get_results(self)
    103 def get_results(self):
--> 104     coords = self.meta.coordinates
    106     coords = np.concatenate((np.ones((*coords.shape[:-1], 1)), coords), axis = -1)
    107     shifts = np.dot( self.regression_coefficients, coords) + 128 # In pixel space

File ~\Prog\LiberTEM\LiberTEM\src\libertem\udf\base.py:502, in UDFMeta.coordinates(self)
    494 @property
    495 def coordinates(self) -> np.ndarray:
    496     """
    497     np.ndarray : Array of coordinates that correspond to the frames in the actual
    498     navigation space which are part of the current tile or partition.
    499 
    500     .. versionadded:: 0.6.0
    501     """
--> 502     assert self._slice is not None
    503     assert self._partition_slice is not None
    504     if self._cached_coordinates is None:

AssertionError: