Merge pull request #12 from dpo/master

Add opHermitian Operator.
New Operator oPermutation.

Implicitly represent a permutation operator based
on a permutation vector.
New Operator opFactorization.

Derived class used to represent inverse operators
based on an appropriate factorization. Optionally,
iterative refinement is performed.
New Operator opLU.

opLU represents an inverse operator based on the LU factorization.
New Operator opChol.

opChol represents the inverse of a definite operator based on
the Cholesky factorization.
New Operator opLDL.

opLDL represents the inverse of a real symmetric operator based
on the symmetric indefinite factorization.