Mislabeled stability indexes

Mathias Bavay
2018-11-23T20:45:16Z

**This issue was reported by Simon Horton.**

I've noticed several places where labels for Sk38 and SSI are confused (based on Stability.cc Sk38 should be Xdata.S_s and SSI as Xdata.S_4). Some places I noticed:

- Comments for the public attributes in DataClasses.h line 531-534
- Header info when writing PRF profile (AsciiIO::writeProfilePrf line 1322-1323)
- Possibly elsewhere?