Slab depth handling

• Convert existing Cascadia slab model depth slices to single file with depth column; will update format of 2018 NSHM. See ghsc/nshmp/nshms/nshm-conus#28 (closed)
• Update deserialization and grid loading to include depth.
• Add max-width to slab-config.json (cleanup and adoption of model will involve removal of max-depth). May be able to refactor slice model to use max-width.
• Separate grid-config.json and slab-config.json deserialization