Code style cleanup

[GenevieveBuckley]

Closes #226
Also addresses most of the points in #101 (except for moving the function definitions out of the __init__.py files)

In this PR, I have:

• Removed any from __future__ imports
• Ran isort on all the files in the repository
• Removed the assert dask statements from the top of several test modules. (I believe they are superfluous, because if dask is not available there will be an ImportError while pytest is collecting tests, so I don't think we'll ever reach that line anyway? If I'm wrong and they're in for a reason, let me know and I'll reverse this change)
• Code style cleanup: Always use import numpy as np
• Code style cleanup: Always use import dask.array as da
• Code style cleanup: Always use import scipy.ndimage
• Code style cleanup: For clarity, use import dask_image.ndfilters instead of import dask_image.ndfilters as da_ndf
• Code style cleanup: flake8 formatting

[GenevieveBuckley]

If the CI checks all pass, I'll merge this.