#23 Tidy-up whilst code reading for mobility heuristic
#23 1st step (of many) for heuristics
#23 Missing files from last commit
#23 Continuation of heuristic refactoring
#23 Move, fix and increase performance of basic statistic classes

1. Move

ScoreInfo -> SampledStatistic
CorrelationInfo -> PearsonCorrelation

2. Fix

SampledStatistic.getStdDev is fixed to calculate it correctly.

3. Performance

Keep totals (rather than averages) until required.
#23 Minor tidy-up of HeuristicScoreInfo
#23 Fix bug in standard deviation calculation

Piece heuristic now identified in Checkers & Pentago but not C4 (as Steve would expect).
Some additional games.
#23 Merge all piece-heuristic function into a single class
#23 Tweak piece correlation threshold after fixing std. dev bug
#23 Heuristics checkpoint

Getting expected results in...

- Checkers (wp+wk vs bp+bk, nothing for blank)
- Breakthrough (white vs black)
- C4 (disabled)
- Pentago (disabled)

Wouldn't expect piece heuristic to be valuable in C4 or Pentago because there's no way of taking a piece and/or placing more than one piece per turn.
#23 Finish refactoring (for now)
#23 Finish refactor-heuristics