Add tool to print label statistics

[schuhschuh]

See IRTK's labelStats. (CC @soundray)

[soundray]

As noted (#696) I can emulate labelStats with the shell function below, just that it is comparatively slow (15 s versus 0.2 s).

mirtklabelstats () {
    ref=$1 ; tst=$2 ;
    evaluate-overlap $ref $tst -labels -table stdout -precision 6 | tail -n +2
    | tr , ' '
    | while read label tp fn fp tn tpr tnr dsc jsc ; do
        echo $label,$[$tp + $fn],$[$tp + $fp],$tp,$fn,$fp,$jsc,$dsc
    done 
}