Update baseline LRG selection

[rongpu]

This PR updates the baseline LRG selection (assuming it will be the final main survey selection). Changes from SV3 include:
* Change the zfiber faint limit from 21.7 to 21.6
* Change the low-z limit from z>0.3 to z>0.4
* Change the overall density from 800/sq.deg. to 600/sq.deg.
* Remove the LRG_LOWDENS target bit

[coveralls]

Coverage decreased (-0.02%) to 59.193% when pulling fd0c8ea on rongpu-lrg into e59fc85 on master.

[geordie666]

@rongpu: This all looks good to me, so no need for a detailed review.

One request. Can you update the docstring in isLRG() from:

- Current version (03/31/21) is version 15 on `the SV3 wiki`_.

to

- Current version (05/04/21) is version 254 on `the wiki`_.

Also, I think we can remove the LRG_LOWDENS bits from the bit-mask yaml file, as those bits have never been used in official Main Survey target catalogs. If you like, I can deprecate those in my working branch, though.

[geordie666]

Thanks! I'll merge once tests pass.

[rongpu]

Thanks @geordie666. I've made the change in the docstring.

And yes, if you could remove the LRG_LOWDENS bits from the yaml file that would be great.