Create a plan that adaptive performs xafs scans

[canismarko]

Instead of specifying the number of scan up front, this plan will evaluate each scan and continue until some pre-determined quality metric is reached.

@yannachen will design the quality metric @canismarko will implement the plan

[canismarko]

The quality metric should accept 2 ndarrays: xdata and ydata, where xdata is E and ydata is µ(E).