Slight refactor of REG 3

[kylebaron]

This modlib model has never been released. I was looking through the code today, and thought this would be a better pattern to follow if someone picked up the code and wanted to follow. I believe the result is the same from what we had before.

• Don't initialize in $PREAMBLE
• Always initialize when starting a new ID (based on NEWIND), but only if REG==3
• Don't reinitialize every time in $EVENT