Pyaerocom writes non-standard json files which can't be parsed by aeroval-web.

[thorbjoernl]

Describe the bug
Please provide a clear and concise description of what the bug is.

• Pyaerocom version: 0.21.dev0
• Computing platform: NA
• Configuration file (if applicable): NA
• Error message (if applicable): NA

To Reproduce
Steps to reproduce the behavior:

1. Run any experiment that includes NaN values in json.
2. Open the experiment on Aeroval.
3. Aeroval is unable to parse json due to non-standard json files.

Expected behavior
Pyaerocom should produce output files which are standard json which can be parsed by aeroval-web

Screenshots
NA

Additional context
Add any other context about the problem here.