add explanation comment to ABIVERSION define

  int latte_abiversion();

}

// the ABIVERSION number here must be kept consistent

// with its counterpart in the LATTE library and the

// prototype above. We want to catch mismatches with

// a meaningful error messages, as they can cause

// difficult to debug crashes or memory corruption.

#define ABIVERSION 20180622

#define INVOKED_PERATOM 8