Ensure index of sparse matrices is always stored as NumPy arrays.

Previously, they could also be stored as plain lists, which would happen
whenever the COOMatrix objects was initialised with lists. Most operations
still worked ok then, but equality check means something completely
different for NumPy arrays, and ProductSpaceOperator's __getitem__
method relies on this behaviour to look up the desired linear index for
a requested element in the matrix of operators. If the index is a list,
this would give wrong results.

odl/util/sparse.py

@@ -3,6 +3,8 @@
 Sparse matrix representation for creating product space operators.
 """
 
+import numpy as np
+
 __all__ = ('COOMatrix',)
 
 
@@ -27,18 +29,19 @@ def __init__(self, data, index, shape):
             raise ValueError('data and index must have the same length')
 
         self.__data = data
-        self.__index = index
+        self.__row_index = np.asarray(index[0])
+        self.__col_index = np.asarray(index[1])
         self.__shape = shape
 
     @property
     def row(self):
         """Return the row indices of the matrix."""
-        return self.__index[0]
+        return self.__row_index
 
     @property
     def col(self):
         """Return the column indices of the matrix."""
-        return self.__index[1]
+        return self.__col_index
 
     @property
     def shape(self):
@@ -51,4 +54,5 @@ def data(self):
         return self.__data
 
     def __repr__(self):
-        return f"COO matrix({self.data}, {self.__index}, {self.shape})"
+        return ( f"COO matrix({self.data},"
+                + f"({self.__row_index}, {self.__col_index}), {self.shape})" )