Update codes for Euro data

[hamishgibbs]

This looks good, I made the following changes to get the WHO codes assigned correctly:

• Changed none_to_empty_str to recognize any null values with pd.isna instead of just if x is None:. This was because a few columns in the input data had numeric null values (nan) not str null values (None).

• Also added data types to the importing of the EURO who_coding file. The columns prov_category and prov_subcategory were being read in as numeric columns and this was killing the data merging.

• Updated the file name in preprocess.py to EURO from Euro so that we are consistent across the whole routine.

[hamishgibbs]

@todowede, all look good here, feel free to merge when you are ready.