Release 3.4.0

[arporter]

The next LFRic release is scheduled for November and a new PSyclone release will be required ahead of that (~September?).
This issue is to capture functionality that is required for that release.

Feel free to add items to the list:

• Support custom function spaces in LFRic (multi-data fields, different number of layers and data) #868 - field arguments with nlayers and/or ndata
• Adopt fparser 0.2.5 #3570