Enable 'prevent' indicator scenarios

[chacalle]

This is related to the work to get a new set of scenarios for 3Bs https://github.com/v-a-s-a/billy/pull/19. For HEP we determined we need to make scenarios at the prevent sub-indicator level rather than at the indicator level.

• Previously the calculate_hep_components function was both calculating the prevent indicator along with the hep index. This change separates the 'calculate_hep_prevent_ind' function from 'calculate_hep_components' to enable scenarios for hep indicators
• small_is_best was missing for the prevent indicator. Assigned to be false
• Update the add_scenario defaults to allow prevent to get through the scenarios.

[chacalle]

I think the better place to do this is to make this change in the underlying csv.

@v-a-s-a can you point me to which csv you are referring to?