title: How to do faceting with sub-facet and aggregate functions using PySolr
body: <p>I have a facet query working with count but I would like to do a sum on a field on the data already grouped by the facet.field. I have the json facet query working in the browser but no luck with PySolr.</p>
  <pre><code>import pysolr
  import pprint
  
  conn = pysolr.Solr('http://localhost:8080/solr/corename')
  result = conn.search('*:*', **{
      'facet': 'true',
      'facet.type': 'terms',
      'facet.limit': 5,
      'facet.field': 'field_name',
      'f.results.facet.sum': '{!func}sum(daily_amount)'
  })   
  
  pprint.pprint(result.facets)
  </code></pre>
  <p>I would like to do a sum on a particualr field, is that possible in PySolr?</p>