[CALC] Improve multi-precision support, and powers/roots. CORE-8486

[CALC] Improve multi-precision support, and powers/roots. CORE-8486

- Added support for average of squares and mean of squares into statistical functions.
- pop() and push() functions in RPN engines now work with nodes instead of stack units.
- Moved the POW and SQR operations near the operators.
CORE-12766

- Fix number of digits for IEEE-754 constants.
- Show all available digits in exp notation.
CORE-14871

- Update help correspondingly.