`semi_additive` is also defined in MathComp 2

[affeldt-aist]

https://github.com/math-comp/analysis/blob/b1a0e33f32bb2f865372d0963afb22b5ad9eadf3/theories/measure.v#L1296

this will conflict with Nmodule @proux01

[proux01]

They remain accessible as GRing.semi_additive and measure.semi_additive so it doesn't look that catastrophic.

[affeldt-aist]

I didn't mean to sound catastrophic ^_^, I just thought that it might be a good timing to think of a name change, I don't think that the semi_ prefix here is as important as it is for NModule @CohenCyril (no hurry anyway)