Remove or update `ll2sphere_xyz`?

[espg]

We currently have this function inside of Utils:

https://github.com/demiangomez/Parallel.GAMIT/blob/e0dc45bc3f2fb294613bac41cac0b0622e7cdbbc/pgamit/Utils.py#L76-L84

The function as written:

1. Does not specify or document the input order for lat and lon coordinates (which 'L' is which??)
2. Ignores elevation / altitude completely during the transform (only calls columns 1 and 3 of a three column "ell" array)
3. Does a spherical transform, which is potentially in conflict with the ellipsoidal ecef coordinates that are used in pgamit

Is anything using this function? Can we remove it, or should it updated to include the ellipsoid , use altitude/elevation, and have a docstring?

[demiangomez]

We should document it but not remove it, although now it is not being used. PGAMIT has several weaknesses, one being lack of documentation. From the code I see that lla[0] is latitude, since the Z component is r * sin(lla[0]). We approximate the earth to a sphere for certain problems, like Euler pole determinations. Elevation is not important for those cases because the purpose is to convert lat lon to xyz on a sphere without considering height. I will push a commit with documentation for this and maybe implement lla2ecef which is the version that transforms from geodetic coordinates. We currently only have ecef2lla.