Light profiles

[b-remy]

I am opening this PR to follow code updates.

So far, Gaussian profiles are implemented and tested. But only for odd number of stamp sizes.

• [x] test even number of pixels

Idem for Sersic profiles, but also need to take into account truncation:

• [x] test even number of pixels
• [x] test truncation

[b-remy]

We might have converged to something we want!

@EiffL , can you think about any other test or feature to add?