new features and extra flagging

This set of commits has the following changes:
- added a gitignore file
- switched plots of neighbors subtraction to biggles
- updated code to require no flags overall and no flags on specific model used
- implemented best_chi2per option to pick model with lowest chi2per to do subtraction 
- added option require_me_goodfit to require that the object have a good ME fit even when dealing with the coadd
- changed options in yaml file to use dict of dict (cleaner and easier to read)
- added option --model-extra-info to read in an extra fits file with additional flags/info not computed by a previous run of ngmix (This will allow us to use only certain objects to do the subtraction and will give a lot of flexibility in the long run.)

I don't know how you compute the overall flags column in the outputs, so some of the flagging done above might be redundant.