Refactor use of sensor id

[caseycesari]

In various places in the code we reference the sensor id. We could clean-up our use of this by implementing the following:

• Use a const to refer the property. In some cases we use Id and others id.
• In some places we look up sensors by Location instead of id. See if we can use id everywhere.
• Currently the id used as the key for the sensor data, and it is also stored a level down along with the sensor values. We shouldn't need to store it there. This could be addressed by Make changes to sensor state structure #64.
• Generally confirm that the id property is used consistently.

Requires #64