Fixed bugs in Expr class
    - Removed baseExpr field
    - Created Expr object expected to be baseExpr
Updated circular dependency exception message in BasicSolver
Fixed issue with BasicSolver implicitly choosing the underlying
property while aggregating

Example:
    DEFINE ROOM R.treasure as AGGREGATE SUM(REG.monster) in REGION REG;
While computing the value of R.treasure, the solver was implicitly
selecting REG.treasure instead of using the property provided