Goals:

1. Check that income claimed by the bank corresponds to the one claimed by DESY
2. Check that all expenses claimed by the bank can be traced back
3. Check the calculation of the bank
4. Analyse expenses. Dominant ones that could be spared on? Any trends?

Todo:

* statistics month: average for each category, possibly a selection of a time period over which avering is done
* statistics year (for a given category): total; average (line)
* fix crash if no entries after filtering found
* introduce sub-categories? (like car: diesel, parts, fines, regular payments such as HUK/Steuer)
* think about categories. e.g. category unknown into two: know what but no receipt, and don't know at all
* average, line and a number - MAJOR
* 'withdrawn' with a single click - not entering the ID. Not a table but some other system to show expenses; with the table column width is fixed,
   description can't be seen - MAJOR
* test if statistics calculation works ok - MAJOR

Note:
"clean" control since 1.1.2017. Before that sometimes periods with many untracked expenses (esp. cash withdrawals)
otherwise 2016, 2015 and partially 2014 are in the database