name choice for xyz_ATTR constants

[fulv]

I was wondering why the names for the three customization constants were chosen to have a leading underscore:

https://github.com/collective/collective.elasticsearch/blob/master/collective/elasticsearch/es.py#L27

CONVERTED_ATTR = '_elasticconverted'
CUSTOM_INDEX_NAME_ATTR = '_elasticcustomindex'
INDEX_VERSION_ATTR = '_elasticindexversion'

The result of this choice is that they can not be set via the ZMI in the portal_catalog/manage_propertiesForm, because you will get an "invalid name" error. I understand that they can be set via code, as described here.
My use case is a set of several distinct Zope instances connected to a single ES cluster. So each instance needs to have its own index name, and it would be necessary to be able to configure these names TTW.

I have added a few fields to this purpose in interfaces.py, but it would have been easier via ZMI.