dev/core#1409 Remove net_amount from Addtional Payment form

dev/core#1409 Remove net_amount from Addtional Payment form

This is causing a validation problem. We used to have an issue on the contribution form which we eventually
resolved by removing net_amount as it's best calculated anyway

In order to make this changed I had to ensure Payment.create adds the net_amount & had
to do a couple of tweaks for the test to pass