Plot multiple spheres

[BobbyCephy]

plot_sphere can also be called with multiple centers, so the sin cos meshgrid does not have to be recomputed.

[AlexanderFabisch]

I am currently a bit busy, but will take a look soon.

[AlexanderFabisch]

I edited the PR and merged it to develop-3. It will be part of release 3.0.0. Thanks for your contribution!

Some changes that I made:

• included the example script in the gallery
• added the function to API documentation
• added a unit test
• fixed default values