Update slab model to use predefined point source depths

In order to use SLAB2 depth, first convert existing slices to single file and test.

changed milestone to %Finalize draft NSHM 2023

added 2023 ERF Hazard-Disagg labels

assigned to @jaltekruse

mentioned in issue ghsc/nshmp/nshmp-lib#173 (closed)

Resolved in:
!52 (merged)
!54 (merged)
!55 (merged)
!56 (merged)
!58 (merged)

closed