fixing graph division
Merge pull request #558 from kreczko/fix_graph_division

fixing graph division
use asrootpy in List.__iter__ and ObjArray.__iter__
Merge pull request #559 from ndawe/master

[MRG] use asrootpy in List.__iter__ and ObjArray.__iter__
ratioplot template
add fill_value argument to Hist.divide
RatioPlot: set tick lengths
RatioPlot: draw horizontal lines in ratio pad
_limits_helper: don't raise ValueError and instead return reasonable values if min=max
templates: center ratio plot y-axis title