Implement regr_sxx, regr_syy, regr_sxy and corr functions as UDAF

We have a requirement to implement the regr_xxx functions for Columnstore. The list of functions to implement did not include regr_sxx, reg_syy or regr_sxy. These functions seem to be part of most other database offerings and should be part of ours.

Also, the corr(x, y) function appears to be in everyone's library as well.

These functions are trivial to implement within the UDAF API. Writing test cases will take longer than implementation.