Add Pearson R coefficient of Correlation

Specifically https://en.wikipedia.org/wiki/Pearson_product-moment_correlation_coefficient

Probably should be added to InfluxQL, but I am not sure how that would work with needing to consume two fields, and there not being join support in InfluxQL yet.

The issue names InfluxQL but no files, tests, or entry points. Start by tracing how InfluxQL functions consume fields and reviewing the existing join limitations; done should include a decided design and a working Pearson product-moment correlation feature with coverage for its two-field input.

Written by the indexing model from the issue text.