DPH options updated

DPH options updated
- '-Odph' is now '-O2 -fsimplifier-phases=3 -fsimplifier-iterations=20'
- The new option '-fdph-none' is the default; it indicates that no DPH
  backend is selected and is the only valid option if the DPH libraries
  are not installed.  If vectorisation is attempted with -fdph-none a
  suitable error message is generated.
- Hence, '-fdph-par' (or '-fdph-seq') needs to be explicitly selected
  when using vectorisation and when linking vectorised code.  (There
  seems to be no elegant way to avoid that.)