Create a generic combined daily data provider

[brandoncharnesky]

There is an opportunity to make combined daily data providers more generic, as well as allow options that are specific to combined data sources.

For instance, the combined-steps data provider: It queries all three data sources, returns the data from each, then applies a max function, and returns the data as though it was from a single source.
For the combined-resting-heart-rate data: It queries all three data sources, returns the data from each, then applies an average function, and returns the data as though it was from a single source.

There can be a generic version of this type of combined data provider. This would also allow the specific individual data sources to be configured (ie Garmin and Google, but not AppleHealth), and the combine function to be specified. There is also some overhead in how these data sources are registered, especially regards to their availability checking functions, that could be simplified as well.