Implement Assertions on Readings

[iain-anderson]

Assertions are an attribute in a device resource's value PropertyValue which specify a string value which the result is compared against. If the comparison fails, then the result is set to a string of the form “Assertion failed for device resource: , with value: ”, this also has a side-effect of setting the device operatingstate to DISABLED. In the case of the single device /command endpoint, a 500 status code is also returned. If the all form of the /command endpoint is used an assertion failure will return a 200 status, and it's the responsibilty of the client to check each reading for assertion failures.

As assertions are operations on results after they've been transformed into the string to be passed in the Reading, assertions cannot as currently defined, be applied to binary readings which result in an array of bytes vs. a string result in the Reading.

[iain-anderson]

Implemented in #54