Optimize corr_matrix()
Optimize corr_matrix()

Correlation matrix has two important properties:
- it is a symmetric matrix
- the values on the diagonal are always 1.0

These two properties can help reduce the times of invoking .corr() from n^2 to (n^2-n)/2 (cut by more than half)
julioasotodv/spark-df-profiling,julioasotodv/spark-df-profiling