Refactor slab sources to use depth in spatial PDF
Addresses tasks in #173 (closed) with the exception of Cascadia 2018 slab conversion. Subset of slab sources used in test model were converted and tests passed without modification.
Addresses tasks in #173 (closed) with the exception of Cascadia 2018 slab conversion. Subset of slab sources used in test model were converted and tests passed without modification.