fix channel checking for dataset-write (#208)

Author: jstriebel

wkcuber/api/MagDataset.py

@@ -105,7 +105,7 @@ def _assert_valid_num_channels(self, write_data_shape):
             ), f"The number of channels of the dataset ({num_channels}) does not match the number of channels of the passed data (1)"
         else:
             assert (
-                num_channels == 3
+                num_channels == write_data_shape[0]
             ), f"The number of channels of the dataset ({num_channels}) does not match the number of channels of the passed data ({write_data_shape[0]})"