Associate pixelization operators with pixel I/O

[tskisner]

There is a non-unique mapping from pixelization operators to I/O routines for a given type of pixelization. We should have pointing operators return the I/O routines to use, so that the mapmaking and map scanning code can just use the correct function. As part of this refactor, we could generalize the map scanning operators so that there is one operator for scanning maps / masks from either memory or a file.