make valgrind-clean

* Field::impl was in a half-way state: allocated by new as a
  shared_ptr, but freed by psfex.  Converted it to a raw pointer
  managed through psfex.
* Removed Psf::_owner as it was unused.
* Set Field::wcs->wcsscalepos.
Merge pull request #2 from PaulPrice/issues/2

Test is not valgrind-clean