derivable-parametrized-function.hh: fix jacobian.
visualization: improve test coverage.
tests: improve test coverage.
tests/result.cc: fix assert.
tests/problem.cc: test for dense and sparse functions.
problem.hh: fix BOOST_MPL_ASSERT_MSG error for abstract types.

While compiling the Ipopt plugin, the following compiler error appeared
in BOOST_MPL_ASSERT_MSG:

  error: invalid abstract parameter type ‘roboptim::GenericDifferentiableFunction<roboptim::EigenMatrixDense>’

This was fixed with an idea coming from:
https://svn.boost.org/trac/boost/ticket/1814
problem.hxx: more BOOST_MPL_ASSERT_MSG fixes.
tests/CMakeLists.txt: fix CHECK_PREFIX usage (valgrind).
Merge with latest changes from master.