Mostly changed Term to use a Number, Expression, and String
Not finished, the Term constructor (that does parsing (or delegates it))
isn't started. Moved isNumber to Number.
Bugs remain, but parser for Term basically finished.
More work can be done on it after #18 is worked out.
Added VariableList to handle Term bodies
Getters to come. Project still containst errors from modifying term, but
it's not done so fixing those compile errors would be premature.
Largly re-wrote Term. Plan to delete TermList.
A bunch remains, but it's better than it was. Now it should parse
2xy2(3x+5) as having a coefficient, two terms with one of them squared,
and an expression.
Fixed bug, added parsing of an expressions's powers
Made almost everything throw MalformedInputException
Worked a bit more on Term (toString() and filling powers with 1)