-some minor fixes to documentation:
	-removed alpha from the units described in fit_linear_diffusion_data(), as alpha should always == 1 for linear (normal) diffusion
	-docstring for mean_square_displacement_by_species() now indicates that a list of trajectory file paths can be provided rather than just a single traj file path
	-fixed a typo in the Examples section of the docstring for above function
	-changed the xlabel of the plot in the docstring of the above function to indicate that these are window sizes