Iterate over coverage pixels in the mask

[elvinpoole]

The healsparse implementation in PR #440 sometimes calls hsp_map.valid_pixels

One should generally iterate over coverage pixels using hsp_map.iter_valid_pixels_by_covpix or hsp_map.valid_pixels_single_covpix for better memory use and to allow parallelization

TXPipe should especially do this when getting valid pixels in the mask, which I expect will be the highest resolution mask TXPipe uses