move separate_multiplicative key outside sampling key in runcard

[RoyStegeman]

This fixes vp-setupfit since there separate_multiplicative was not parsed from the sampling key and thus it was not found. This was broken in #2107

This also resolves #2106 by removing the sampling namespace all together, though since we never use it we may also want to drop the separate_multiplicative keyword all together.

[scarlehoff]

This also resolves #2106 by removing the sampling namespace all together,

This solution works for me.

though since we never use it we may also want to drop the separate_multiplicative keyword all together.

But given we already have the option to be able to change between the two modes, we might as well keep it. It might be useful to test (and to compare with 4.0).

[andreab1997]

Looks good to me, thanks for this!

[RoyStegeman]

I don't think you can use the same name for user-input and for the result of a parse_ or produce_ function