Add asserts to tests

[cdeil]

Currently most of the tests just have print statements at the end: https://github.com/tisimst/mcerp/blob/master/mcerp/tests/

Those should be changed to have assert statements, or numpy.testing.assert_allclose for floating point numbers.

Plotting tests should either be removed or moved into a separate test function. E.g. the test in test_lhd.py currently is skipped in continuous integration because the scatterplot_matrix package isn't available:

https://github.com/tisimst/mcerp/blob/2bb8260c9ad2d58a806847f1b627b6451e407de1/mcerp/tests/test_lhd.py#L31

I can't find scatterplot_matrix on PyPI. @tisimst - Could you please comment what / where this is? Should it be added or that plotting test removed?