config update: planning and pitfalls

<Usecase config>
1. User creates original grid
2. User creates grid sample set by setting 2D downsample
3. User focuses on first random FOV.
4. User starts 1st iteration on all samples across sub-grids
5. Computer analyzes results from 4 and output 4-type classes
6. Computer puts Type-1 from this iteration to scan queue
7. For each of Type-2 ~ Type4, repeats 3~7, when new children branch out, recursively do the same, whenever the children are complete, i.e., no further Type-2~4 out, go up for parents till it seeks back to the top node, whose parent is NaN;
8. Note, there is supposed to be identifiable opds under temp folder whenever fov_verify() is called. they have to match the scan tree-traversing sequence, e.g., it's always type2->type4, and depth-first

* pitfall
- type 2 and type 3 can't miss 2+ times,
because the depth range is pretty confined: top only and
bottom only, with scan-depth being rather narrow, ensure that FOVs are pretty aligned vertically, so if one of them works, all the other in the same group should also work.
- so making a recursive structure for type2 and 3 is overkill, there will be at most only two levels.