Add alias to gaussian_filter

[K-Monty]

Closes #192

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2. If the pull request adds functionality, the docs should be updated. Put
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[GenevieveBuckley]

Here are some comments, although you might like to wait a bit for responses to this comment before trying it out.

1. Here's an example where we alias a function for deprecation reasons.

sum is an alias of sum_labels

dask-image/dask_image/ndmeasure/__init__.py

Lines 716 to 719 in 91fe6e1

	def sum(image, label_image=None, index=None):
	"""DEPRECATED FUNCTION. Use `sum_labels` instead."""
	warnings.warn("DEPRECATED FUNCTION. Use `sum_labels` instead.", DeprecationWarning)
	return sum_labels(image, label_image=label_image, index=index)

Obviously, there's no need for a warning in this PR. Maybe the docstring could be as simple as """Alias of 'gaussian_filter'."""

2. The second step is to add the alias function name to __all__

[K-Monty]

@GenevieveBuckley Thanks a lot for your comments! It seems like there is no objection(s) in the comment, so I'd given it another try.
By the way, do I also need to pay attention to the checks? For my last pull request, it said "All checks have failed".

[GenevieveBuckley]

Thanks @K-Monty, I've added some comments for you.

The checks all seem to have passed with your most recent commit, so I wouldn't worry about why they failed earlier unless we start seeing that again.

[GenevieveBuckley]

Ok, this looks good to me now!

I'm going to leave this PR open for another week, just to give @jakirkham & others a bit more time for this discussion around design decisions. If there are no strong objections in a week, then we'll merge this here (feel free to ping me if I forget).