Update gate locations due to terminus retreat

[mankoff]

See GEUS-Glaciology-and-Climate/Sentinel-1_Greenland_Ice_Velocity#7

Options:

• Update baseline product definition, perhaps with recent ice extent map.
• Update only where needed at baseline period and (1x; regenerate product). Perhaps this is just Sermeq Kujalleq (examine everywhere else to see if needed).
• Update gates at each timestep
• Other... ?

[mankoff]

5000 m gates vs. 2019 ITS_LIVE basemap.

Only Sermeq Kujalleq seems to be poorly located.