Additional checks to PlotAno

As reported by Virginie Guemas, PlotAno checks could be improved by:

adding additional check on the arguments :

if (max(nchar(sdates)) != 8 | min(nchar(sdates)) != 8 ) { stop ("sdates should be formatted as YYYYMMDD") }

I would include that check between lines 119 (end of check on the dimensions) and line 120 (start of axis limit definitions)