Provide virtual cumulative counters

[tzwenn]

Devices like the MCP have multiple channels, yet we might to now the cumulative power draw of all channels (or if multiple are attached the sum).
Implement a new virtual data source, that adds the power draws/energy results from a given list of other counters.

Caveats to consider:

• Caching results, so the original data sources are not queried twice if the users wants the virtual as well as the original data
• How to handle different timestamps / available access frequencies?