Added Exceptions for Matrix class
Updated documentation for Matrix exceptions
Updated CholeskyFactorization.java to zer out upper triangular entries
Added GaussTransformation.java, which allows for creation of elementary elimination matrices.
Added LUFactorization.java
Added forward and backward substitution solvers, ForwardSubsitution.java and BackwardSubstitution.java
Added solvers which use LU factorization.
Added a TODO list