Add comparison amount on Balance Sheet and Income Statement
It is a very common use case to compare those reports with a previous version like previous year for Balance Sheet.
The general design can be discussed on https://discuss.tryton.org/t/comparison-fields-for-report/99
For Balance Sheet, I propose to allow to compare with a different date with will be by default the previous year.
For Income Statement, I propose to allow to compare with a different fiscal year (or the same) and over a different optional period.