Add PointingProvider

[mreineck]

No description provided.

[ziotom78]

Hi @mreineck , sorry for the delay in my comments. This implementation looks very handy!

One minor point: I see that PyPointingProvider::pyget_rotated_quaternions allocates the array where to store the result. Would it make sense to provide a similar method where the result array is passed as argument? In this way one could allocate the array once and then iterate the call to pyget_rotated_quaternions while re-using the same array, reducing the time needed for memory allocation.

[mreineck]

Sure, that can be done easily!

[mreineck]

An implementation-detail question for @tskisner: what is the internal storage format for quaternions in TOAST? w,x,y,z or x,y,z,w? I'm trying to avoid confusion wherever possible, so I'd like to use the same order.

[mreineck]

I should perhaps note that functionality which is very similar to the PointingProvider can be emulated using scipy.spatial.transform (I just noticed this while writing unit tests). It appears to be roughly a factor of 10 slower, but this is not necessarily a big deal. So if there is preference for using scipy here, that's absolutely fine with me.

In any case I'll use the (x,y,z,w) ordering for quaternions in my code to be compatible with scipy in that respect.

[tskisner]

Hello @mreineck , sorry for the delayed response. TOAST currently uses "scalar last" format, but there have been requests to support "scalar first" as well (used by Boost quaternions). Our issue for tracking that (which I just updated) is here: hpc4cmb/toast#335

[mreineck]

Thanks! Scalar-last it is then, at least for the time being.