Create Lagrange DM to get global section for fault preconditioner.
Fix small compile bug.
Add DM for subfields in Field. Refactor Field metadata.

Refactor _metadata and _subfieldComps in Field into _metadata (field
only) and _subfields (metadata + auxiliary info).

Rename Field::petscSection() to Field::localSection().
Add Field::globalSection().
Fix bug. Remove superfluous DMDestroy().
Cleanup by using Field::globalSection().
Merge branch 'baagaard/fix-custom-faultpc'

* baagaard/fix-custom-faultpc:
  Cleanup by using Field::globalSection().
  Fix bug. Remove superfluous DMDestroy().
  Add DM for subfields in Field. Refactor Field metadata.
  Fix small compile bug.
  Create Lagrange DM to get global section for fault preconditioner.