Updated Column class to store Reference rather than String

Updated several other classes to reflect this refactoring
	modified:   src/development/hierarchical_split/HierarchicalSplit.java
	modified:   src/internal/parser/containers/Reference.java
	modified:   src/internal/parser/containers/condition/BaseCondition.java
	modified:   src/internal/parser/containers/expr/Expr.java
	modified:   src/internal/parser/resolve/Column.java
	modified:   src/internal/parser/resolve/Result.java
	modified:   src/internal/parser/resolve/constraint/BasicSolver.java