Track (and potentially lift) restrictions

The current implementation has some obvious restrictions that may not be necessary. These revolve around values for origin and target shower having to be the same, like core and atmosphere. I am not sure it is actually necessary to have this. So we could experiment with allowing more freedom in that regard, and have for example a origin atmosphere and a target atmosphere.

If they cannot be lifted, these limitations should be clearly communicated in the documentation (see also #8 (closed) )