new faint limits to BGS

[qmxp55]

Just a small change for BGS. We have updated the faint limits to r< 20.175 in DECaLS and to r<20.22 for BASS/MzLS in order to get a target density of ~1400

[geordie666]

Thanks, @qmxp55. Can you also:

• update the changes.rst file to describe this PR.
• update the Main Survey wiki page.
• update the doc string in the isBGS() function to read:

Current version (05/06/21) is version X on `the wiki`_.

instead of:

Current version (03/29/21) is version 13 on `the SV3 wiki`_.

where X is the version to which you update the Main Survey wiki page.

[qmxp55]

Hi @geordie666, I've made the corresponding updates. Let me know if there's anything else that needs to be done.

[geordie666]

Thanks @qmxp55. This looks good to me, and the changes are simple enough that I'll merge this PR once tests pass.