feat: new observe_coeval function

[steven-murray]

Add a function to easily "observe" a coeval box. This has many similarities to observe_lightcone (and perhaps we could merge some of the code) but also some simplifications (especially due to having only one redshift).

I opted to go the "simpler" route of only having to supply the Observation and the coeval dimensions, rather than the full sigma_uv like we do for observe_lightcone. I could be argued out of this, or argued into applying the same to the observe_lightcone function.

[nikos-triantafyllou]

So this is just to save time from having to compute every sigma at every slice?
One could argue that we would never want this, except if we want to run some test.

But it could be the same as if we provide a coeval, and an array filled with the same redshifts for the observe_lightcone

[steven-murray]

@nikos-triantafyllou yes, it should be essentially the same as running observe_lightcone and passing in an array of the same redshifts except that the wedge filter is also different -- we don't have to do any chunking or rolling or anything, because we have a periodic box.

So, in the end it is faster and more convenient.

[nikos-triantafyllou]

Looks good, I am still a bit concerned if it complicates things, but we can remove it later based on the final workflow we decide on.