Started syn-addition of peroxyacid to alkene (SAPA)

Based on EA2A.

Added SAPAModel.java as a copy of EA2AModel.java.
In SAPAModel:
- removed HCl
- Fixed the position of both of the alkene's central carbons, instead of
having the top carbon be the center about which the reaction rotates
- Moved the alkene up in Y by half a bond length, so the origin
initially falls in the middle of the pi-bond