Add support for trade party contact details

[neuged]

This adds TradeContact as a model allowing us set:

• BT-34 seller electronic address
• BT-49 buyer electronic address
• BT-41 contact point
• BT-42 contact phone
• BT-43 contact email

together on TradeParty for both buyers and sellers.

This somewhat conflicts with the prior addition of TradeParty.person_name which already triggers the creation of a ram:DefinedTradeContact group in the xml. Setting person_name directly is still supported here as an alternative to giving the TradeContact directly, so this is fully backwards compatible.

Note also that #54 also allows to add email and phone details, so that might be a different road you might want go, adding the details directly on TradeParty.

This is partial support for the XRechnung CIUS. I will open an issue shortly and link this PR from there.

Any comments appreciated. Please feel free to make any changes you might need. Feel free to reject this as well.