Add alt output format of slice of predictions

[RenatoGeh]

This patch provides an alternative output format. Previously, retrieving
an array of predictions was worst case quadratic time, as each cluster's
observations would have to be compared with the original observations
and then assigned their designated cluster. This commit resolves this
issue by simply returning the already generated internal slice of
predictions from Partition.
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This is a use case that I have. I need performance and to be able to preserve the original observations order, so I'm sending this as a PR. Totally understand if you don't want to merge this, as it doesn't use your muesli/clusters format.

I wasn't sure on the name (PartitionPoints), so feel free to change that (or provide suggestions for a v2).

[coveralls]

Pull Request Test Coverage Report for Build 42

• 11 of 11 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+4.03%) to 42.623%

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Totals
Change from base Build 41:	4.03%
Covered Lines:	52
Relevant Lines:	122

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💛 - Coveralls

[RenatoGeh]

Hi,

I would like to know whether a merge is still on the table, as I need to make the necessary changes in my own repositories.

Regarding Travis, a quick search shows that this error is probably due to a type alias from Go's experimental image package. Type aliases were introduced in Go 1.9, and all Travis builds from 1.9+ are successful, meaning that's probably it.

Thanks,
Renato