Make it possible to easily add new auto-ROI algorithms

[raacampbell]

The main user-facing functions, such as autoROI, should stay superficially the same. The interface should change as little as possible. But there should be the option to have different backends so different algorithms can be used for finding the brain.

Made a reference dataset today. So will now start the long process of moving the existing algorithm into it's own directory structure.

[raacampbell]

The existing algorithm is moved to the module dynamicThresh_Alg with common functions remaining in autoROI. Verified that the refactored code generates the same test results and also runs BakingTray autoROI in dummy mode. 75f7b04

Next job is to try adding a new algorithm -- the chunked CNN -- and see how easy this is and whether more code needs to be moved out to a separate module.

[raacampbell]

This is now mostly working