- use refPtr to simplify some logic.
- avoid copying field if an average will be used
- initialize geometric fields with a uniform value instead of Zero
- minor tweak of method names

- apply bugfix #1889 (longer description elsewhere)

- cleaner code, more similarity with unique_ptr

  Now
      if (ptr)
      if (!ptr)

  instead
      if (ptr.valid())
      if (!ptr.valid())

- some support for "uniform" bool fields. Calculating an averaged
  value for a boolField does not work very well, but we simply define
  that the field average is 'true' when more than 1/2 of its values
  are true. Not exactly true, but allows templated definitions to work
  smoothly.

- additional output method writeValue().
  This outputs the single (uniform) value or the first value of the
  field.

- polymorphic field/result