Fix json nan-serialization

[thorbjoernl]

Change Summary

Use ignore_nan=True when writing json, which writes compliant json and fixes an issue where aeroval-web can't parse generated json.

Related issue number

metno/pyaerocom#1288

Checklist

• Start with a draft-PR
• The PR title is a good summary of the changes
• PR is set to AeroTools and a tentative milestone
• Documentation reflects the changes where applicable
• Tests for the changes exist where applicable
• Tests pass locally
• Tests pass on CI
• At least 1 reviewer is selected
• Make PR ready to review

Additional Notes

This introduces a potential issue in that the data written by aerovaldb will not be the same when read in for nan values. aerovaldb writes nan as null, then reads null as None when reading. AFAICT this matches the old behavior of pyaerocom as seen in the write_json and read_json methods respectively, so I am leaving this as is for now.