Feature: probability time periods for hazard maps

[chrisdicaprio]

Users want PoE in 100yr hazard maps:

• Add additional PoEs to the grid database #428
• Add additional PoEs to drop-down list. List would look like, e.g.:

2% in 50 years
2.5% in 50 years
...
2% in 100 years
2.5% in 100 years

• Kororaa API support for additional PoEs (if necessary) GNS-Science/kororaa-graphql-api#84: PARKED, perform PoE conversion in Kororaa, no need for API support
• REMINDER: @chrisdicaprio needs to generate the grids