Loading FieldPerp variables

[johnomotani]

Currently FieldPerp variables are not supported, we just skip them when loading https://github.com/boutproject/xBOUT/blob/1f2090e0167b5147a0489889151a76c625b47fc9/xbout/load.py#L700-L707

Following discussion here https://github.com/boutproject/BOUT-dev/pull/2102#pullrequestreview-496349751, I think what we want to do is:

• check if a variable is a FieldPerp as we already do (checking it has x and z dimensions)
• get it's yindex_global attribute
  • if yindex_global == -1 then ignore it. -1 means it is not a valid FieldPerp - there might be data in it, but we do not have to support getting it automatically. The user can get it out with boututils.Datafile or similar if they really want it.
  • if yindex_global >= 0 then read the FieldPerp. Probably the best thing to do would be to rename it appending the value of yindex_global to the name, because a FieldPerp variable with the same name might have been written at different positions with different values of yindex_global. For example a sheath-boundary heat flux might be written at both lower and upper divertor targets in a double null configuration using the same variable.
  • if a FieldPerp is saved with the same yindex_global on processors with different y-processor index PE_YIND this should be an error.

For reading, need to work out how the requirements above can be implemented with xarray's combining methods. Hopefully it might 'just work'. I'm not sure right now what happens to variables that are present in some files but not others.

If we find that a FieldPerp was read at only one, unique yindex_global then I guess we could remove the suffix from its variable name, but that would require more special-case code so I'd vote against unless someone wants that feature badly.

When writing out the FieldPerps:

• save() creates a single file, so I think we have to write with yindex_global still appended to the variable name
• create_restarts() needs to work out which files to write the FieldPerp() into (the ones where yindex_global is within the grid or boundary cells) and needs to remove the suffix from the variable name.