GnuCash Cashflow reports

[Peter Broadbery]

Hi,

I've been updating the cashflow report to remove some rather bad
behaviour for large data sets.  The changes take an example  5000
transaction report from around 100 seconds down to around 2 seconds.

Assuming that there is interest in getting this into gnucash I've
pushed it to https://github.com/pbroadbery/gnucash/tree/pab/reports-maint
The code is against the 2.6.9 (maint) branch,  I've tested against my
accounts and not noticed any discrepancies, and added a few unit tests
for the core calculation.

Peter