masking algorithm

[Gioacchino Wang]

1. add <masks> block in xml parameter input
2. allow different mask map for each observable at each frequency and Nside
3. if not all outputs have mask, no mask is used in LoS integration
4. if not all outputs have mask, a pre-check of mask and integration longitude/latitude limit should be performed while loading masks, to ensure the unmasked region is not ruled out by lon-lat limit
5. if all outputs have mask, a global mask should be generated for minimal masking in LoS integration, and in this case, lon-lat limit should be inactivated.
6. if all outputs have mask, a global mask generation should also consider the lowest Nside in integration and among all provided mask maps, dive to the highest Nside and generate the minimal masking map. (rule 6 is to ensure that no unmasked region for any observable is ruled out, converting low-res mask to high-res mask is fine, while converting high-res mask to low-res mask through HEALPix function does not satisfy out minimal masking principle)

[Gioacchino Wang]

mask algorithm implemented based in HEALPix up/down-grading