Update Cascadia

• Build new spreadsheet of magnitudes, rates, and figures; add along strike slip calculation.
• Create cluster and GEA17 source-tree branches
• Update Cascadia README in source model

The necessary information to do the above is in Teams: NSHMP > CONUS > 2023 > Cascadia Source Model
See also ghsc/nshmp/nshmp-lib#160 (closed)