Do not hard-code default column for plots in HTML tables.
Do not hard-code default column for plots in HTML tables.

Previously always the second column was used when a user clicked on
"Quantile Plot" or "Scatter Plot".
Now we use the first numeric column, if there is one.

This fixes #260.

martin-neuhaeusser/benchexec,IljaZakharov/benchexec,ultimate-pa/benchexec,martin-neuhaeusser/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,dbeyer/benchexec,martin-neuhaeusser/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,IljaZakharov/benchexec,dbeyer/benchexec,IljaZakharov/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,martin-neuhaeusser/benchexec,IljaZakharov/benchexec,dbeyer/benchexec,ultimate-pa/benchexec