Improve spreadsheet

[aptiko]

The unit tests check the results of calculations against @nmalamos's Excel files. However this is not mentioned anywhere. We should insert comments in the unit tests that explain from which spreadsheet (and which lines of this spreadsheet) the unit test takes its values. Before that, we need to define names in the spreadsheet to make formulas readable, add variable root depth stuff, and find a way to share the spreadsheet file.