Add 'partial YES values' formula to schema

[DinisCruz]

When adding support for partial YES values (see #104) I don't think that all activities will need a percentage value (i.e. 1% to 100%). In fact, some might only need 2 or 4 values (25%, 50%, 75% or 100%).

Add these mappings to the schema file so that we can control what type of data should be provided for a particular activity

The UI should reflect this