Adopt common code for setting "scalingFactor" and "value" keys

[pp-mo]

As for example #205, but some general functionality.
Probably something like scalefactor_and_value(value, min_factor=0, max_factor=7)

A fair bit of work to apply this everywhere, naturally.
But we use currently use fixed values almost everywhere, which is just a limitation waiting to bite someone (i.e. values saved with unexpectedly limited precision)

[pp-mo]

See also : #211