Attempt to address issue #410

1. In order to change functions to be inline, I have to remove internal attribute for StatsHelper function. Otherwise I got error message. "error FS1113: The value 'mean' was marked inline but its implementation makes use of an internal or private function which is not sufficiently accessible" It's similar to https://github.com/hedgehogqa/fsharp-hedgehog/blob/master/src/Hedgehog/Range.fs#L94. Hence I renamed StatsHelper to be StatsInternal.
2. Basic stats functions and level stats functions shall work for int, decimal and float now. The function outputs are still float no matter what input type is.
3. I cannot make expanding window and moving window functions work. Both cases call applySeriesProj. But applySeriesProj has to be internal as it's calling other internal functions. I assume it's because of series.VectorBuilder as it's an internal member. @tpetricek Any idea how to get around it?