Tests error out or fail without additional dependencies

[taldcroft]

Describe the bug

• Tests do not run without installing h5py.
• Tests do not pass without installing an additional dependencies.

To Reproduce

$ conda create -n martini python=3.7 
$ conda activate martini
$ pip install pytest
$ pip install -e .
$ pytest
...
============================================== short test summary info ==============================================
ERROR tests/test_martini.py
ERROR tests/test_write.py
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 2 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
================================================= 2 errors in 0.11s =================================================
$ pip install h5py
$ pytest
...
============================================== short test summary info ==============================================
FAILED tests/test_martini.py::TestMartini::test_preview - ModuleNotFoundError: No module named 'matplotlib'
FAILED tests/test_martini.py::TestParallel::test_parallel_consistent_with_serial[True] - ModuleNotFoundError: No module named 'multiprocess'
FAILED tests/test_martini.py::TestParallel::test_parallel_consistent_with_serial[False] - ModuleNotFoundError: No module named 'multiprocess'
FAILED tests/test_martini.py::TestGlobalProfile::test_preview - ModuleNotFoundError: No module named 'matplotlib'
FAILED tests/test_martini.py::TestGlobalProfile::test_view_spectrum - ModuleNotFoundError: No module named 'matplotlib'
FAILED tests/test_sources.py::TestSPHSource::test_preview - ModuleNotFoundError: No module named 'matplotlib'
FAILED tests/test_sources.py::TestSPHSource::test_preview_save[pdf] - ModuleNotFoundError: No module named 'matplotlib'
FAILED tests/test_sources.py::TestSPHSource::test_preview_save[png] - ModuleNotFoundError: No module named 'matplotlib'
FAILED tests/test_spectral_models.py::TestParallelSpectra::test_parallel_spectra[GaussianSpectrum] - ModuleNotFoundError: No module named 'multiprocess'
FAILED tests/test_spectral_models.py::TestParallelSpectra::test_parallel_spectra[DiracDeltaSpectrum] - ModuleNotFoundError: No module named 'multiprocess'
==================================== 10 failed, 887 passed, 7 xfailed in 57.08s =====================================

Expected behaviour

Tests should pass or be skipped if optional dependencies are not installed.

Additional context
Review for pyOpenSci/software-submission#164.

[kyleaoman]

Closed by #59