Breaking:
- evaluate_stability drops evaluation_mode; mode carries it.
- mode is assembly or elements only; pinned, welded, contact and the
  multi_body_ spellings are errors.
- assign_mass reads density and mass in the document's own units. An
  imperial document now reads a bare number as lbm/ft3, sixteen times the
  mass, with no error. Read before upgrading.
- assign_mass total_mass_kg is what the scope weighs, not what the call
  wrote; assigned_mass_kg is the latter.

Fixed:
- open_file opens before it closes, so Rhino is never left with no document,
  and it fails rather than leave a different document active than the one
  it says it opened.
- assign_mass with overwrite=False reports the mass each object kept.

Nine documentation errors corrected, found by re-running every guide page
against this build.