ENH: improve exprResult handling

- 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.