#11 arinvoicesum.php SQL errors

[Peter Kools]

The current implementation of arinvoicesum.php runs
into a m by n problem in the query on line 26 under
certain circumstances. The file included in this patch is a
replacement which eliminates the m by n issues by
decomposing the multi-join query into a single sum multi-
join query with three additional queries to get the other
sums required for the invoice report. I left the original
query commented out in the source code for reference.

[Peter Kools]