Add utility callback for filtering records by existential flags

[thisisaaronland]

For example, take this which is in the go-whosonfirst-pip-v2 package and make it a high-level shared thingy for use in other packages...

type ApplicationIndexerOptions struct {
        IndexMode         string
        IsWOF             bool
        IncludeDeprecated bool
        IncludeSuperseded bool
        IncludeCeased     bool
        IncludeNotCurrent bool
}

func NewApplicationIndexer(appindex pip.Index, opts ApplicationIndexerOptions) (*index.Indexer, error) {

        cb := func(fh io.Reader, ctx context.Context, args ...interface{}) error {

                var f geojson.Feature

                if opts.IsWOF {

                        ok, err := pip_utils.IsValidRecord(fh, ctx)

                        if err != nil {
                                return err
                        }

                        if !ok {
                                return err
                        }

                        tmp, err := feature.LoadWOFFeatureFromReader(fh)

                        if err != nil {
                                return err
                        }

                        if !opts.IncludeNotCurrent {

                                fl, err := whosonfirst.IsCurrent(f)

                                if err != nil {
                                        return err
                                }

                                if fl.IsTrue() && fl.IsKnown() {
                                        return nil
                                }
                        }
 
                         // and so  on...

[thisisaaronland]

Have a look at:

• https://github.com/whosonfirst/go-whosonfirst-index/blob/master/query.go

It might be worth considering the callback-equivalent of io.MultiWriter