Rename & refactor cubic solver

Renamed CalcRoots to CalcCubicRoots, refactored common code into FindCubicRoot
Created function SortRoots to extract code that has been copy-pasted in various files into 1 place.
Refactor to use SortRoots function
Merge pull request #28 from waqqasdadabhoy/master

Rename & refactor cubic solver