Changing name of functions in struct step

According to some remark about the name of the fields,
the names are changed to process, undo and clean.

Change-Id: Ie529b48c933aaae58514620d0a7c81a26f0d4477
Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>
adding C++ interface

Change-Id: I3f61ca1f886027a45b0d031905662203802844f9
Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>