_XarrayStorage cannot handle general cases

`_XarrayStorage` does not support the merging between activations with different neuroid coords. E.g, one activation array with channel coord and another with channel_x and channel_y coords in addition.

Also, it is not clear how to extend the coord `neuroid_num` when the activation array of a new layer is added.

Start by reading the _XarrayStorage implementation and trace how activations are merged when a new layer is added. Reproduce the cases described with differing neuroid coordinates, including channel versus channel_x and channel_y. Done means these activations merge correctly and neuroid_num extends consistently.

Written by the indexing model from the issue text.