quantities: Make mean() more generic.

[Sbte]

I ran into this while trying to pass the where argument to numpy.mean().

[LourensVeen]

Looks fine to me. It does extend the AMUSE API to match all of the numpy API (and it means we'll evolve with numpy if they change) but we're doing that already for median(), so I'd say that's alright.

Is there any particular usage you have that you want to add a test for?

[Sbte]

Thanks for the review Lourens. I added a test. I'm not sure why the tests are numbered instead of using meaningful names like test_mean, but I followed the same pattern.

[rieder]

Looks good to me!
Only comment I have is that it's best to give tests a more descriptive name, the numbered tests is a relic from when we used nosetests instead of pytest.

[Sbte]

Ah, perfect. I changed the name.