Reading type need to handle more than just numeric data

[chanskw]

Current, we assume that Reading is a numeric reading from a bedside monitor.
In some cases, the reading can be just a text value from the device. And that data comes in from the same channel. We need to figure out what to do with this type of data.

[chanskw]

@cancilla I suggest that we add a "description" attribute to the Reading type. This allows us to encode non-numeric value into a reading. This is an optional attribute that can be an empty string. What are your thoughts?