Missing support for "erfcinv" and "erfinv" math functions in TICK scripts

The Math functions included in this list [1] do not include the inverse function for both the "erf" and "erfc" functions. It would be useful to include "erfcinv" and "erfinv" to the supported list.

[1] https://docs.influxdata.com/kapacitor/v1.5/tick/expr/

Start with the linked TICK expression math-function documentation and trace the existing erf and erfc entries to their implementation and tests. Confirm where supported functions are registered, then add coverage and documentation for erfcinv and erfinv; done means both functions are accepted in TICK scripts and appear in the supported list.

Written by the indexing model from the issue text.