Add warning when applying factor mask or regrid mask with non-existent ISO3 codes

When applying a factor mask or a regrid mask using an ISO3 code, HERMES does not currently verify whether the provided ISO3 code actually exists in the user’s country mask. This means that if the ISO3 code is misspelled or if the country mask is incomplete, the operation proceeds silently without notifying the user. As a result, the output may be incorrect, and the user has no immediate indication that something went wrong.

@ctenaI propose that HERMES emit a warning whenever an ISO3 code is used that does not appear in the loaded country mask.

FYI: @mguevara