Use IZUGFeRDExportableItem.getCalculation instead of LineCalculator constructor

[WorkProg]

No description provided.

[jstaerk]

1. why? and
2. can you please fix the conflicts?

thanks!

[WorkProg]

why? and

IMO doesn't make much sense to have that function and not use it, concretely I was looking into issues with the sums and was confused my changes didn't affect anything because the function is not used.

[WorkProg]

@jstaerk to give some more context, we have aggregation at a different place in our system that calculates the row totals and it is more important, that the numbers line up with the values we print on e.g. the PDF than that the value is consistent according to unitPrice * amount = rowTotal.

Ideally we'd want to be able to provide mustang with all values it needs to put on the e-invoice and do the calculations our selves.

[WorkProg]

@jstaerk, sorry to bother you again. Do you know if this can be merged soon? We are currently blocked by this in our E-Invoicing integration.