Major refactoring; use AlgebraicParticle instead of Equation in most
cases
Not compiling
Move tests and core into same package, different directory
Reach compilable state after refactoring except for CombineLikeTerms
Fix bug on signs
Fix unit test for number
Refactor Term.isParticle()
Refactor Root.isRoot()
Fix compilation errors caused by bad throws declarations
Document AlgebraicParticle
Add Variable and unit test
and small debug code in AlgebraicParticle
Add toString() to a couple classes, integrate Variable class with rest
of system