feat: add halo start indices to partition_metadata

[TomMelt]

In order to send halo regions via MPI in the dynamics, we need to know the size of the domains and the starting indices of the halo regions.

This PR adds functionality we need to parallelize nextsimdg dynamics.

The following improvements have been made.

• minor tweaks to DomainUtils to add helper methods
  • get_width
  • get_height
• added Edge enums and array edges for more readable loops over neighbours
• reworked neighbour checking to be more consistent between periodic and non-periodic neighbours
• add functionality to compute the starting indices for the halo regions in neighbouring domains
• replaced loops over NNBRS to use the new edges enums
• minor fix - ensure that std::cerr messages are followed by exit(EXIT_FAILURE).

TODO

• [x] update integration tests to reflect additional outputs (halo_starts)