color-dependent galaxy clustering

[yymao]

As discussed in #10 and #63, we'd like to make sure the catalog has reasonable color-dependent galaxy-galaxy clustering signals.

This should be relatively easy to implement: we can use the current galaxy-galaxy clustering and just implement color-selection. We can use SDSS data as validation data. Validation criteria TBD.

• code to reduce mock data
• code that works within DESCQA framework
• validation data
• validation criteria

[yymao]

Implemented in #100. Given the small sky area of protoDC2, having validation criteria is not meanfuling at this moment. We can revisit this if we have snapshot data / a larger sky area.