Query style interface into Consumption data

[philngo]

It would be helpful in some instances to have query-style interface into ConsumptionHistory objects. Right now, the only way to access data in sequentially.

It would be nice to be able to efficiently make the following types of queries:

• What was the consumption on date X?
• What was the total consumption over datetime range Y?
• What was the average consumption over datetime range Y?

[philngo]

This would come for free if we used pandas series for storing consumption time series data. The subsequent speed increases (also for weather sources) may make it worth the substantial refactoring that switching to pandas would require.

[philngo]

ConsumptionData objects are built on top of pandas, which allows this to be done quite easily and naturally.