Minor issues in `AddCoordinateChannels` transform

[Nic-Ma]

Is your feature request related to a problem? Please describe.
There are several minor issues in the AddCoordinateChannels transform that need to refine:

1. It's utility transform not spatial transform, because actually it changes the channel dim not spatial dims.
2. Better to use spatial_dims as arg name to be consistent with other transforms instead of spatial_channels.
3. Should count spatial dims from "0", don't consider channel as 0 for "spatial dims".

Would be nice to refine it in the next release.