Rename SS_thd50 and SS_thd85 to SS_thd1 and SS_thd2

[MaxGhenis]

I think SS_thd50 and SS_thd85 are the only policy parameters with their default values in their name. This also diverges from SS_percentage1 and SS_percentage2 which are set to 0.5 and 0.85, respectively. Renaming the thresholds to SS_thd1 and SS_thd2 would be more consistent.

The variable descriptions also specify 50% and 85% in them, unlike other thresholds.

[TishaJhabak1014]

@jdebacker Is this issue solved??

[jdebacker]

@TishaJhabak1014 This issue is still open. It will make a change that is backwards incompatible. I've created a new v4_development branch to merge the changes related to this issue into whenever someone decides to tackle it.