Hardwired "p0" in some files

[jpolton]

E.g. 'profile_errors_p0_????%02d_????.nc'%(month) in EN4_postprocessing/merge_monthly.py

This is not quite $MOD = P0.0 in config.sh but could perhaps be flush out and replaced with a variable.

[rdPatmore]

I've noticed this. In my local version of the crps code I have removed the p0 label from the file naming and added it to the nc attributes. The reasoning being that file stucture is already segmented by p*.