FieldFactory: variable scope working

Variables without a section (e.g.  "a", "nx") are looked
for in the current section, then all parent sections.

Variables with sections (e.g. "mesh:nx") start from root section,
and look only in Options->getRoot()->getSection(mesh), and not its parents (for now).

May need refinement, depending on use cases, and what people find
most intuitive.