Decompose across multiple panels

### Are there any linked Issues or Pull Requests?

#138

### Brief description

Modify the existing partitioning strategy so that it can partition multiple panels (either 3 sets of 2 or 2 sets of 3) into uniform rectangular domains.

### Further details of the issue.

Modify the existing partitioning strategy so that it can partition multiple panels (either 3 sets of 2 or 2 sets of 3) into uniform rectangular domains. This will mean that the number of mpi ranks needs to only have a factor of 2 or 3 instead of 6 as a base requirement.

Using this option means that code that assumes all owned cells are on the same panel (we think only the extended mesh in the transport code) will not give the correct results here.

As part of this change I have also fixed the plotting script for the baroclinic wave on azspice where the figures were not coming out correct

This issue is a port of the SRS ticket https://code.metoffice.gov.uk/trac/lfric_apps/ticket/1070