[Update]: "IMP" entry is formatted incorrectly (not grouped by network)

[rt17603]

What needs updating?

The "IMP" entry is not formatted correctly as this doesn't have a network: https://github.com/openghg/openghg_defs/blob/main/openghg_defs/data/site_info.json#L1411C1-L1419C1

Network value should be added and this updated to a nested dictionary

[rt17603]

@EricSaboya - would you be able to update the "IMP" entry to include a network please? The openghg reader gets confused with this entry otherwise as this is expected to be grouped by network and nested. Thanks!