added functions to correct for extinction and v6 cuts

[myamamoto26]

• Added functions to compute dereddened magnitudes.
• Added a function to add and change columns and save in a new file.
• Added V6 cuts (new i-band mag 24.5 cut, size cut, updated pgauss cut etc.) and new mask locations.

[myamamoto26]

To-Do: adjust i-band magnitude cut.

[beckermr]

We need to double check a few items and write a few tests before we merge.

• figure out new i-band cut (from @myamamoto26's comment)
• write tests of new helper functions to compute asinh flux from mag etc.
• double check the asinh mag stuff is ok to use with the reddening stuff with Eli

[beckermr]

I chatted with Eli and we need to apply the dereddening to the fluxes directly, not the lupton mags.