-I've added scipy and MDAnalysis to requirements.txt
-I'm now working to generate an artificial xtc file that can be used for initial unit testing of the new mean_square_displacement_by_species() function
-wrote new module to produce the xtc file: test_data/generate_artificial_xtc_diffusion_testing.py
-I've suppressed print statements in mean_square_displacement_by_species()
-wrote new unit test for mean_square_displacement_by_species() [and it seems to work great after a few failures and adjustments to the test code]