Fixing Classification Metrics

- Classify.ecl
There was a bug in FP_Rate_ByClass when working with big datasets:
"Error: System error: 10099: Graph[329], workunitwrite[338]: Dataset too large to output to workunit (limit 10) megabytes, in result (name=spillI), Master exception"

Changed all the rate calculations to use CrossAssignments accumulated data instead of
accumulate J (DiffRec  recordset) with different conditions for each calculation.