There was a bug in the current behavior where the rule `frequencies` was calling `config["frequencies"]["narrow_bandwidth"]`. This resulted in --narrow-bandwidth to be fixed to whatever was specified in parameters.yaml. However, we were relying on build-specific settings to differentiate behavior in the all-time vs 6m vs 2m vs 1m builds, ala:

```
frequencies
    global_1m:
      narrow_bandwidth: 0.019
```

This commit fixes this issue and allows overriding of narrow_bandwidth in parameters.yaml by build-specific settings. It also provides a genuine default (equal to augur default) so that parameters.yaml doesn't have to always specify narrow_bandwidth.

This removes `recent_days_to_censor` from GISAID/21L and open builds to match removal from GISAID builds.

This follows recommendation in issue #1131 to use the pattern

frequencies:
  default:
    min_date: "6M"
    narrow_bandwidth: 0.038

This use of "default" extends just to min_date, max_date and narrow_bandwidth. This behavior is now documented in parameters.yaml as well as workflow-config-file.rst.

The specification of frequencies parameters in builds.yaml now follows this pattern.