Co-ordinate aware sphere object with Persistence

[tmolteno]

This issue is to track the development of a new co-ordinate aware sphere. The sphere should be able to manage it's own transformations between elevation-azimuth as well as celestial co-ordinates.

• [ ] Every pixel has co-ordinates
• [ ] Introduce the concept of the phase centre of the sphere. This is the pixel that represents the phase centre, and it will have celestial and el_az coordinates.
• [ ] Add methods to transform between co-ordinates celestial_to_elaz, elaz_to_celestial
• [ ] Create a storage format for spheres that can be reused. This is likely to be vtk.
• [ ] Shift drawing to a new utility (disko_plot) that draws over sphere's. Also exports as FITS e.t.c.

Issue #7 is relevant here. This will allow plotting of sources in either coordinate system (satellites, or celestial objects)

[tmolteno]

Persistence:

So we can't use healpix in general for persistence because healpix can't work with very narrow angle fields of view. This is where the unstructured mesh sphere's come in. The .vtk format (see paraview) is a standard mesh format that isn't entirely ideal at this stage because mesh points have to be three dimensional to represent the surface of a sphere. This means that we have to come up with our own persistence format. Some options:

• [ ] JSON - this is text and therefore too bulky. It has advantages in that it is human readable.
• [ ] HDF - binary, and already used for data and visibility objects by TART
• [ ] The MOC standard](https://ivoa.net/documents/MOC/)

[tmolteno]

Current Plan:

Change the name Sphere to FieldOfViev, and refactor disko into two modules

• disko.fov For the field-of-view classes FieldOfView, HealpixSphere, HealpixSubsphere, MeshFOV, AdaptiveMeshFOV
• disko.image: For the radio astronomical imaging algorithms

This will eventually become two separate projects, with the source drawing e.t.c. being shifted to the fov object.