Improved performance

[MichaelSchreier]

In order to increase the impact of numba's JIT all the heavy lifting from the _distances() method was moved into its own method that can be compiled far more efficiently.

As a result calls to fit() result in a runtime decrease of about 30 to more than 90(!) percent, in particular on repeated calls with larger datasets.

The "new" _compute_distance_and_neighbor_matrix() method is set to nopython=False only because test_data_format was behaving weirdly, which I couldn't resolve.

[coveralls]

Pull Request Test Coverage Report for Build 89

• 24 of 28 (85.71%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.3%) to 98.689%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
PyNomaly/loop.py	24	28	85.71%
<!--	Total:	24	28	85.71%	-->

Totals
Change from base Build 86:	-1.3%
Covered Lines:	301
Relevant Lines:	305

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💛 - Coveralls

[MichaelSchreier]

I've reduced the use of numba to the one function call where it is actually beneficial to the performance and made the use of it entirely optional. Unfortunately the conditional imports are reducing the coverage and I didn't find a way to prevent the import of numba in a test case.

[vc1492a]

@MichaelSchreier Thanks, this is great! I'll check out the PR and will include this in version 0.3.1 along with some other needed changes.