Update calc.py

It is an algorithm for a calculator capable in total 8 operations:

1. Addition '+'
2. Subtraction '-'
3. Multiplication '*'
4. Division '/'
5. Taking powers '**'
6. Showing a percent of c of a '%'
7. Taking a root of any given number. In other words, taking 1/c powers 'root'
8. Find the greatest common divisor of two numbers 'gcd'