uf_addmarginal deprecate beta selection

[behinger]

If one selects a beta-field in uf_addmarignal, then only that field gets the marginal added, but not the other.
e.g.
ufresult.beta
.beta_nodc

specified with 'betasetname','beta'
will result in
ufresult.beta+marginal
.beta_nodc

This is a bit confusing / inconsistent. The only potential usecase I could see, is taht someone copies the .beta field and wants to keep both .beta_marginal and .beta .

I think we could deprecate this feature