reorganised regrid code to check output before merging

Sugata Narsey
2020-06-02T15:39:13+10:00

I've made two changes:

1. added a manual fix to sellevel if required by user.

2. re-arranged the code to check if output file exists FIRST, before mergetime. Speeds up the script a lot, since merging time takes a long time unnecessarily for all models and experiments.