More precise Ohms_Law and OhmsLaw_AC() Class

- Shorten the check cases to take out unnecessary evaluations.
- Remove unneeded parentheses in variable definitions to make code more readable (Python's precedence of arithmetic operations makes certain parentheses unneeded).
- Change values of all variables to be floats (not just rational numbers). More precise calculations for float input and provide more consistency throughout.
Merge pull request #136 from CherryJam/develop

More precise Ohms_Law and OhmsLaw_AC() Class