Small fixes in interpolation.py

Some variables were not interpolated onto WOA grid in the 6th cycle.

Reviewing the script interpolation.py I've found two issues that stood out:

1. Comparison with mixed types

Line 43: |if cycle == 6:

The variable 'cycle' contains a string, not an integer. Such a comparison will fail (FALSE)

Changed to:

         |if cycle == '6':

2. Non-empty empty list

Line 59: |    vars_zonal = ['']

Such an assignment doesn't drain the list but creates an empty string in it, wich is actually an element.

Changed to:

         |    vars_zonal = []