Encapsulate Split::Algorithms at our own module to avoid explicit calling rubystats everywhere

[andrehjr]

• Encapsulates RubyStats on its own module. Avoiding calling it from other places in code. Split just needs the BetaDistribution.
• Warns if matrix is not installed (stblib made it into a gem). Although this should be fixed on rubystats upstream, for now, we try to warn users and to make our own specs pass.