Fast cythonized inverse array of unsorted, unique indices

[lilyminium]

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Is your feature request related to a problem?

PR #3368 creates an inverse index array of unsorted indices with this Python loop:

            mask = np.zeros_like(self.ix)
            for i, x in enumerate(indices):
                values = np.where(self.ix == x)[0]
                mask[values] = i
            self._unique_restore_mask = mask

Describe the solution you'd like

This could be done very easily and much faster in Cython instead, in the lib module.

Describe alternatives you've considered

Additional context

[SanjanaSogimatt]

Hey, I am new to open source so it would be great if I can help to solve this issue.

[IAlibay]

@SanjanaSogimatt please do go ahead and open up a PR with code to solve this issue.

[SanjanaSogimatt]

Hey actually I'm new to open source. Can you give me some suggestions on how to get started? It would be really a great help

[IAlibay]

@SanjanaSogimatt please have a look at our userguide on contributing to MDAnalysis: https://userguide.mdanalysis.org/stable/contributing.html

If you open up a pull request which addresses this issue we can guide you through the process of merging your code into our codebase.

[lilyminium]

I added a comment elaborating on this problem here: https://github.com/MDAnalysis/mdanalysis/pull/3433#issuecomment-940087191