Add numpy to requirements.txt

[hmacdope]

As discussed on #33, we can make NumPy a required dependency.
Probably best to use the same version as MDA.

[hmacdope]

I can't assign you for some reason @BFedder.

[BFedder]

I had added NumPy as required dependency as part of #33, and have now changed it to require the same version(s) as MDAnalysis

[BFedder]

This is now merged. However, with MDAnalysis moving to numpy>1.20, it might be worth to do the same here, but this would mean dropping Python3.6. Do we want to do that here?

[IAlibay]

Imho no - MDA will enforce it's own subselection of numpy & python because it lives downstream. It's fine for panedr to support a wider range until it becomes absolutely necessary to restrict to a newer version.

[BFedder]

@hmacdope, are you happy for this issue to be closed now that numpy>1.19 is in requirements.txt?

[hmacdope]

Yep all good!