Make EIGEN2_SUPPORT fully compatible with Eigen2 and tested

Submitted by Benoit Jacob

Assigned to Nobody

As discussed on the thread: Eigen2 to Eigen3 Migration Path

To do:

when EIGEN2_SUPPORT is defined, relax const correctness at least in Map constructors
add back all Eigen2 features when EIGEN2_SUPPORT is defined
if some Eigen2 feature has an unsolvable conflict with Eigen3, consider options. Could ignore if it's a very minor feature, or add a separate EIGEN2_FULL_SUPPORT_BREAK_EIGEN3 option.
import the eigen2 test suite into the eigen3 test suite, though we could drop some variants of tests for lighter compilation.
do not aim at ABI compatibility with Eigen2, that's a lost cause (different template parameter default values, different index types). Instead aim at API compatibility so people can just flip a flag and recompile.