compute tripolar boundary metadata #86

TODO:

• add new logic to compute tripolar boundaries
• add periodicity type enum/flag/string to the metadata netCDF so that we can implement correct boundaries in nextsim (match convention with nextsim and call it structure_name. Check for match in nextsim)
• (optional) add decomp args (i.e., command run to generate metadata) to the netCDF. Could be helpful for recomputing metadata (addressed by Add command arguments to metadata #88)