Add PermutationMatrix
fix test due to matexpr/matrix interactions
MatrixExprs inputs to PermutationMatrices
permutation identity for matrixexpr inputs
PermutationMatrix inputs any iterable

As a result it now interacts well with combinatorics.Permutation objects
fix permutation doctest

Needed Matrix call to make the MatMul explicit
test args for PermutationMatrix
add PermutationMatrix to docs
absolute imports in PermutationMatrix
obj.__class__(*obj.args) for PermutationMatrix
property -> @property def return
HadamardProduct infers element type