change N/A value to 0

[kimlee87]

Preprocess downloaded files to change N/A value to 0.

Note: double check if 0 makes sense in all cases before changing.

[kimlee87]

Replacing all NaN values with 0.0 can be problematic, as the appropriate treatment depends on the variable type.

For instance, setting missing totals (e.g. total precipitation) to 0.0 may be reasonable, whereas a missing temperature value does not imply a temperature of 0.0 degree.

Therefore, we leave this issue unresolved for now.