Trimming should be aware of model configuration

There are differences between model configurations which should be taken into account when trimming:

• Nests may not have ghost points at all.
• ORCA025 differs in how the folding in the North is done
• Other (non-Orca) grid configurations are possible.

I think, this could be easily done by passing a keyword to preprocess_orca() which then would determine how the selection in trim_and_squeeze() is done exactly.

Tasks:

• provide standard model configs
  • "GLOBAL" (trim 1 at all boundaries)
  • "NESTED" (do not trim)
  • default to "GLOBAL"
• Compare case-insensitive