Explicitly store alternative billing preference

account.billingPreference is either:
* an empty string for none
* 'address' to use alt. billing address
* 'eInvoice' to use eInvoice data
Set account.billingPreference in import

If eInvoice data exists --> eInvoice
If a billing address exists --> address
Otherwise empty.
Use account.billingPreference in proe creation

So print billing.address or eInvoice data as requested

Also, don't completely fail if there is no address, could be that an eInvoice is being used or maybe the receiving system has an address for the account.
Merge pull request #27 from kavi-fi/store-billing-preference

Store billing preference