expand scope of style expressions

[sbidoul]

It is currently possible to define style expressions at the kpi level.

Eg: 'some style' if profit < 0 else 'some other style'.

That works fine for reports where the only difference between columns is the time period.

For reports made possible in version 3 where columns are more specialized, it makes sense to have style expressions apply at the column level (eg to apply something such as 'red' if cell_val < 0 else ''.

Such generic style expressions should be defined on styles (so one can define styles such as red if negative), and then applied at the column level with the implementation of #14.

[github-actions]

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