[IMP] account: invoice analysis pivot quantities

In the report "Invoice Analysis", the view under the report is grouping
lines by invoice and also by product on this invoice.

Thus, quantities in the pivot view are not the real quantity of invoice
lines, but of invoice lines with different products.

eg:

invoice a:
- line 1: product X
- line 2: product Y
- line 3: product X

in the report there would appear to only be 2 invoice line, whilst there
is in realy three (this information is accessible via "# of Lines").

closes #11954
opw-676651