Improve speed of calculations

[morremeyer]

We can improve the speed of calculations as follows:

• Research if we can use shopspring.decimal together with storing numbers as int only (probably by storing the exponent separately or even using fixed point?)

If not:

• Store numbers as integers only

Then, calculations can be done by the database with e.g. SUM() operations and the results only need to be transformed. This should increase performance by enabling us to use fewer queries, e.g. when calculating the month sums