Handle G18 input data

Right now grb2scmi.sh (and likely other scripts) use G16/G17 tags to decide whether to generate E or W tile sets. Future G18 is intended as GOES-West.

• create a feat-g18-support branch
• make necessary script changes
• isolated manual testing
• make a merge request to master
• merged-stream testing, preferably with live data
• update documentation
• demonstrate on SAP server
• merge to master