Improve spike/outlier detection

[squaregoldfish]

The current High Delta check is a very simple affair. It defines a delta in units and a timespan, and checks the rate of change between any two consecutive measurements to see if it exceeds this rate. This can be improved by the creation of two separate routines:

Long term drift
Define a unit difference and a long time period. Measurements that are that period apart will be checked to see if the change is greater than the specified unit difference. We may need to specify n measurements to be averaged at each end to take account of short term noise.

Spike detection
A new approach to spike detection: Get the deltas between measurements across the entire data set, expressed as a change in units over time. Calculate the standard deviation of these deltas. A spike can be defined as any delta that's outside three standard deviations.

[squaregoldfish]

These would replace the existing HighDelta routine.