Compile errors in boost-eigen branch

The boost-eigen branch does not compile for me. This is after the merge from kloudkl  (8d894f0) (I never tested it before that).

boost::math::nextafter should only be parametrised on one type (or so says gcc here, and what I read on the boost docs).

Also, the tests for test_random_number_generator reference mean_bound and sample_mean but they need to be referenced as this->mean_bound (again, that's just what my gcc thinks).

This branch contains those two fixes. After this, everything compiles and the tests run fine (except for some GPU tests)