Add statistic checks

[felixbur]

The explore module computes statistic measures for correlation, but some of them have assumptions, e.g.
PCC assumes that both variables are normally distributed.
Add a check and a warning if requirements are not met.

[bagustris]

Instead of warning, adding distribution/statistic checks into reports/results would be useful too. It also supports bias checking (if the data is not normally distributed or skewed in particular characteristics).