ENH: Update Pre-Processing UI to improve BVR workflow (#141)

Improves user workflow in the Pre-Processing module, and refactors
the BVR configuration file generation UI. The changes include:

* 'Volume Node' and 'Output Directory' at top of UI were swapped
* 'Trial Name' moved to the config section as the ''.cfg field
* Segmentation generation UI redesigned to better convey the two
  separate generation methods
* 'Adavnced Options' renamed to 'Default Subdirectories', moved 
  below the 'Segmentation Generation' section, and rendered as
  collapsed by default
* 'VRG Resolution' fields from 'Advanced Options' moved to the
  config generation section
* 'Generate Config' section: added file and path selectors to
  specify the camera calibration files, radiograph root directories,
  and volume files. Also added 'Voxel Size' fields that are
  automatically populated based on the selected input volume node.
* Elements in .ui file were edited to be properly indexed and named

Co-authored-by: Amy M Morton <[email protected]>