[GNC] Improved Transaction Report Summary

[flywire]

I hacked
https://github.com/flywire/gnucash/blob/Subtotal/gnucash/report/trep-engine.scm
to Use trep-engine lowest shown grouping for detail (
https://github.com/Gnucash/gnucash/pull/1624#issuecomment-3636584747 ).

The challenge to the community is to break it with real world testing:

   1. Download the report (using GitHub icon for Download raw file) and
   save it in GNC_CONFIG_HOME -
   https://wiki.gnucash.org/wiki/Configuration_Locations#GNC_CONFIG_HOME
   2. Edit or create your GNC_CONFIG_HOME/config-user.scm to add the line:
   (load (gnc-build-userdata-path "trep-engine.scm"))
   3. Run a transaction report and select the option(s) on Sorting tab Show
   subtotals only

To be clear, this is all about using any combination of Primary and/or
Secondary subtotals to summarise transactions how you want them. It retains
the original functionality of reverting to transactions with no active
subtotals which I consider a fundamental design bug for another day.

Any comments appreciated.

Regards