Filtered Groups and additional keywords/functions

[hilburn]

Is your feature request related to a problem? Please describe.
Not really a problem, so much an annoyance - but I'd like to be able to group by battery type (using Battery Notes it's the battery_type attribute) automatically rather than having to define which of my connected devices uses AAs, which use CR2477 etc in the group as a list
Additionally I would like to then display the sum of batteries needed to replace the ones that are in the group

Describe the solution you'd like

1. Getting an entity list by applying a second layer of filtering within the group - could replace the current simple min/max limits
2. Adding a "sum" keyword, and the ability to define entity attributes other than state for the keywords: e.g. {sum:attribute.battery_count} would provide the sum of that attribute within the group

Additional context