#86 negative bin counts in histogram undefined?

[Peter Doerfler]

lti::histogram (and possibly its subclasses) implicitly
assumes that the bin counts are positive. This is used
e.g. by calculating the numberOfEntries as the sum of
all bin counts.

I'm not sure if negative bin counts make sense or not.

If no, this should be documented and enforced by
assert() in debug mode.

If yes, the code is very buggy, but fixable by taking
the absolute values of bin counts for some calculations.

Either way, this needs improvement in versions 1.9.x
and 2.0