title: Indexing decimal values in zope catalog
body: <p>I'm using Plone 4.2.5 and dexterity</p>
  
  <p>I have added a field to a custom content type and would like to index one of the fields. It's a decimal field. Preferrably I'd like to be able to query the catalog using 'greater than' or 'less than or equal to' type logic for the index. Is this possible? What type of index should I use?</p>