Petascope_WCPS add handlers for statistics var_* functions() as in Rasql

[Bang Pham Huu]

Rasql supports statistics functions like:

rasql -q 'select var_pop(c) from test_mr as c' --out string

Then, WCPS also should support as:

for c in (test_mr) return var_pop(c)

[Vlad Merticariu]

var_pop is not part of the WCPS standard, please don't start implementing this until we discuss in the group.