ENH: make precision adaptors modifiable (#2173)

- allows reuse similar to refPtr for wrapping different content.

- additional control for when contents are copied back,
  instead of waiting for the adaptor to go out of scope.

  Eg,

  if (adaptor.active())
  {
      adaptor.commit();
      adaptor.clear();
  }

- static ConstPrecisionAdaptor::get method renamed to 'select' as a
  better description of its purpose and avoid confusion with
  non-static 'get' method.

  Was previously only used within GAMGPreconditioner, but even there
  it is better just to use the ConstPrecisionAdaptor directly.