public Object convert(String fieldName, Inspector value) {
        DocsumField field = fields.get(fieldName);
        if (field == null || ! value.valid()) return null;
        return field.convert(value);
    }
Returns the given slime value as the type specified in this, or null if the type is not known.
Even in a correctly configured system we may encounter field names for which we do not know the type,
in the time period when a configuration is changing and one node has received the new configuration and
another has not.
container-search/src/main/java/com/yahoo/prelude/fastsearch/DocsumDefinition.java