Parallelize T1w conformation step

[effigies]

In the event we need to resample more than 1 or 2 T1w images (e.g. ds031), t1_conform can become a bottleneck, only utilizing one core as it runs nilearn.image.resample_img in series.

This process can be broken down into two steps:

1. Read all images, decide on target zooms/dimensions (requires full list)
2. Resample images (only one image, and target zooms/dimensions required)