const correct in matrix/vector from array
apf::eigen outputs Vector<3> not Vector3

since this is an lvalue, it is stricter
so having it be Vector3 prevents people
from passing an array of Vector<3>,
including the common case of using
a Matrix3x3 object.
adding a test for apf::eigen

this consists of 6 randomly generated
symmetric matrices and their eigenvalue
decompositions as given by Octave.
it checks that apf::eigen gives almost
the same answer (if you want more precision,
start by printing the hardcoded values
with more decimal places).
test the eigen function nightly