MIN: Some minor changes in ErrorMetrics.
MIN: Some minor changes in ErrorMetrics.

Including that (i) pbias is now computed in percent as it should, (ii) pbias
is reported by default, (iii) avoiding reports and plot attempts of no valid
pairs of obs and est are available.

wradlib/wradlib,heistermann/wradlib,heistermann/wradlib,wradlib/wradlib,kmuehlbauer/wradlib,kmuehlbauer/wradlib