sin(0) mode is not always 0 in the disk if Nphi=2

[csskene]

If the phi resolution in a disk is exactly 2 then the sin(0) mode is not zeroed.
For example if I set Nr=1 and Nphi=2 and make a field f initialised with f['c'][:2]=1, then after transforming to grid space and back it is still the case that f['c'][:2]=1. If Nphi=4 then doing the same procedure results in f['c'][1] as 0. Attached is a small script that demonstrates this as Nphi is changed.

disk_nphi2.txt