Include peripheral glaciers

[mankoff]

• Need to update MMB to include peripherals
  • Need ice thickness - where from?
  • Have recent velocities (ITS_LIVE, not PROMICE Sentinel)
  • No historical velocities...
• SMB from some of the RCMs
• BMB ?

[mankoff]

See also https://github.com/GEUS-Glaciology-and-Climate/ice_discharge/issues/29