Allow ad-hoc additional fields rather then explicitly setting a field name as "additional_fields"

[e-lo]

1. it is confusing what it is.
2. what if you have multiple?

[ssmith55]

Agreed. What we had was a placeholder, with the intent of allowing ad-hoc additional fields.

[joshchea]

User defined or custom fields could have any name except the required/predefined GMNS field names for the network object in question. This could be enforced by issuing a warning/error when parsing the data in the validation tool by checking set membership against predefined attribute names.