Add FB_BasicStats for encoders by default (report stdev/average/etc) [ECS-1534]

[klauer]

Expected Behavior

Each encoded stage should have statistics about its current position recorded at all times and exposed to EPICS.
This diagnostic information could be important for diagnosing issues with any stage - and we might not know it until we need it.

Context / environment

The issue calls for statistics on critical axes, but I would argue that we should calculate them for all:

https://jira.slac.stanford.edu/browse/ECS-1534

... [A] lesson learned here is to make sure all critical axes like those for mirrors come equipped with the statistics FB blocks so we are always measuring the noise RMS. Our engineering specifications only call for one test, but it's a sad day when noise suddenly appears and we have no hope to correlate it with some configuration change.

[ZLLentz]

I agree.

We should decide on the default settings for this. Essentially we can pick between:

• how many elements to accumulate in the stats
• whether to recalculate after the array is full or continuously as each element is added