bugfix for empty template cutting

tfields/core.py

@@ -1408,10 +1408,8 @@ class Tensors(AbstractNdarray):
 
         # template
         indices = np.arange(len(self))[mask]
-        # TODO: maybe np.empty instead of inst below. Reason why i copy is
-        # that the template is such marked for where it comes from.
-        # We have a trade-off with small memory consumpiton here.
-        template = tfields.TensorFields(inst, indices)
+
+        template = tfields.TensorFields(np.empty((len(indices), 0)), indices)
         return inst, template
 
     def _cut_template(self, template):
@@ -1449,7 +1447,7 @@ class Tensors(AbstractNdarray):
                     projected_field[nan_mask] = np.nan  # correction for nan
                     fields.append(projected_field)
         if dim(template) == 0:
-            tensors = self
+            tensors = np.array(self)[template.fields[0]]
         else:
             tensors = template
         return type(self)(Tensors(tensors), *fields)
@@ -2413,21 +2411,6 @@ class TensorMaps(TensorFields):
         else:
             return inst
 
-    def _cut_sympy(self, expression):
-        if len(self) == 0:
-            return self.copy()
-        mask = self.evalf(expression)  # coord_sys is handled by tmp_transform
-        mask.astype(bool)
-        inst = self[mask].copy()
-
-        # template
-        indices = np.arange(len(self))[mask]
-        # TODO: maybe np.empty instead of inst below. Reason why i copy is
-        # that the template is such marked for where it comes from.
-        # We have a trade-off with small memory consumpiton here.
-        template = tfields.TensorFields(inst, indices)
-        return inst, template
-
     def _cut_template(self, template):
         """
         Args: