Part one of thickness reservoirs

[kshedstrom]

This is a draft PR for thickness reservoirs. The code creates an object, it doesn't break existing cases, and restarts seem to work. Some questions remain:

• Is this the correct timestepping of them?
• They are turned on by setting either OBC_TRACER_RESERVOIR_LENGTH_SCALE_OUT or
  OBC_TRACER_RESERVOIR_LENGTH_SCALE_IN. Is this what we want?
• This doesn't (yet) handle the SIMPLE_H obc case which could help the layered Kelvin wave case.
• I check for the external variable "DZ" for reading dz. Not sure what we want here either, not that this is completely wired up yet.
• Happy to chat about this if it helps.

[kshedstrom]

Thank you for the thoughtful suggestions. Hopefully they have all been addressed now.