Add statistic calculation operations to `stats` module

[joshiayush]

Order Statistics

• percentile
• quantile

Averages and Variances

• mean Added statistic calculating functions to ai.stats module #20
• median Added statistic calculating functions to ai.stats module #20
• std Added statistic calculating functions to ai.stats module #20
• var Added statistic calculating functions to ai.stats module #20
• varcoef Added statistic calculating functions to ai.stats module #20
• zscore Added statistic calculating functions to ai.stats module #20

Correlating

• corrcoef
• cov