Update gridded seismicity spatial PDFs to V8

Reference issue for updates to spatial PDFs.
CEUS spatial PDFs were updated in !95 (merged)
WUS spatial PDFs were integrated into fault system solution and updated in !97 (merged)

This issue includes graphics associated with the updates.

added 2023 ERF labels

Maps of the 2018 spatial PDFs and the V8 spatial PDFs

mentioned in merge request !95 (merged)

V8 active crust spatial PDFs

For the record, here are maps for grid-system-active (commit 513655d9) of incremental rate for select magnitudes in the gridded MFDs (file active-crust/grid/grid-data/branch-avg-grid.csv) derived from the V8 spatial PDFs.

Added maps of V8 spatial PDFs and the derived gridded MFDs

closed