simplify GENPREFIX name for 1_SIM stage

[RickKessler]

The 1_SIM stage created by pippin has a typical GENPREFIX name such as
GENPREFIX: PIP_ROMAN_STRATEGY01_BCOR52aaa

where the "52aaa" part is corrupted and likely due to a special character (+ or -) in the name. The actual GENPREFIX string does not have to be descriptive and was intended to be something very short such as the survey name. For pippin, I would recommend hard-coding a single PIP string in call cases;
GENPREFIX: PIP
Note that the GENVERSION folder name provides the unique identifier, and thus there is no need to distinguish the prefix used inside the folder.