Corrected bug in the creation of the 4DVAR normalization file

[arango]

Corrected a bug in def_norm.F that affected leaving the definition mode during the file creation. An END IF was out of place. Many thanks to Gordon Zhang for reporting this problem.

I also corrected a typo in normalization.F and allowed def_ini.F and rp_def_ini.F to open the NetCDF even if there are no variables to add. This allow us to run 4DVAR in unclean initial conditions files during debugging.