tidepool atlas averaging is very broken

[bbfrederick]

Describe the bug
Atlas averaging in tidepool is not fully implemented. Averaging to use masked overlay values in the numerator (anything not in the mask is set to zero), but is divided by the number of the voxels in the entire region, not just in the mask.

To Reproduce
Steps to reproduce the behavior:

1. Load a data set with many voxels missing in the valid mask
2. Click on any Region Summary metric other than raw data
3. Region values are anomalously low

[bbfrederick]

I've made some fixes to this, but got sidetracked a bit by the need to support multiple versions of MNI152 space now that I'm using fmriprep. As soon as fmriprep allows my processing to work fully with the MNI152Nlin6Asym output space, I may be able to simplify the code quite a bit.