Add test for same value repeated

[dschristianson]

Potential QAQC test.

For example, a sensor gets stuck and / or gap-fill fills with mean value

Be careful with soil temperature for sites that have snow -- values should hold fairly steady around 0. There may be other variables like this.

From HC: I'd suggest have this check in data qaqc, as it gets into variable-specific tests, and mostly like requires curator's judgement.