Improve cof memory management

[RoelBouman]

This PR adds functionality to COF to allow for the selection of one of two methods:
"fast" which will use the old calculation using the full pairwise distance matrix.
"memory" which will use a new method, which only calculates distances when needed, thus saving on memory at the price of speed.

This PR follows up on issue #315

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[coveralls]

Pull Request Test Coverage Report for Build 2063

• 97 of 98 (98.98%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 95.834%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyod/models/cof.py	25	26	96.15%

Totals
Change from base Build 2052:	0.06%
Covered Lines:	5429
Relevant Lines:	5665

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💛 - Coveralls

[yzhao062]

Hi Roel,

Could you elaborate a bit on the definition of fast and memory option? Since we need to add this to the documentation.
The example looks like:

Thanks for contributing!
Cheers,

[RoelBouman]

Thanks for the feedback! I've indeed forgotten to update the documentation properly. The latest commits should solve this.

[yzhao062]

Perfect. Quick eyeballing looks good to me and it also satisfies the unit tests. I will merge it for now for some local tests.
I will take care of minor things. If you like, I could add you here (https://pyod.readthedocs.io/en/latest/about.html) who makes a reasonable amount of contribution to PyOD :)

[RoelBouman]

Great! I would certainly appreciate being added to the about page. The info for that would be:
Roel Bouman (Ph.D. Student @ Radboud University):

• Joined in 2021
• LinkedIn (Roel Bouman) <https://nl.linkedin.com/in/roel-bouman-18b5b9167>_

[yzhao062]

Great! Will update the documentation accordingly.