Leaner code & active max h collection

A series of updates:

• Free foreign particles and the mesh before i/o operations.
• Free foreign particles before exchanging the proxy cells.
• Hide the unnecessary fields into anonymous unions inside the cell sub-structures to save memory.
• Collect the max active h of each cell.

The first three updates are currently in use in the Sibelius-dark run. The last change is not used yet for performance improvement but will be used in a future push that focuses solely on using the quantity to speed-up loops. At the risk of confusing things, I added it here already as I was making substantial changes to the cell structure organisation.