ValidateDatacards report for 1-bin shapes

[giacomoortona]

Currently, ValidateDatacards reports a "smallShapeEff" if a distribution has a single bin, since it compares normalised distribution. Although in this special case the effect is intended and the implementation is valid.
It would be better to report this differently, maybe as a warning that one could use lnN instead.
(this is for bookkeeping, I will work on this myself)