ERT-518 - Changed how EnkfFs instances are cached and used in EnkfFsManager
ERT-518: Merge remote-tracking branch 'jpb/ERT-518' into ERT-518-segmentation-fault
ERT-518: Changed the enkf_fs implemenatition: Calling scope must use incref() and decref(). The enkf_fs_umount() function is static and called implicitly from decref() when the refcount == 0.
ERT-518: Changed Python EnkfFS implementation to use incref() and decref(). The python variables holding on to a enkf_fs instance will automatically call incref on it.