ENH: yPlus: support disable of field writing (#2813)

- for simulations where the yPlus is needed for other purposes or
  just for obtaining information on the patches it can be useful
  to disable field writing and save disk space.

  The 'writeFields' flag (as per some other function objects)
  has been added control writing the yPlus volume field.

  If unspecified, the default value is 'true' so that the yPlus
  function object continues to work as before.
  However, this default may change to 'false' in the future to align
  with other function objects.

ENH: wallShearStress: support disable of field writing

- similar to yPlus, the write() method combines writing information
  and writing the fields. The 'writeFields' flag allows some
  separation of that logic.