Adg_?Dim: allow-none for AdgPair and named pairs

Adg_?Dim: allow-none for AdgPair and named pairs

The points of interest are now set only if the input pair is defined.
Previously a NULL value raised an error message.

NULL points must be valid in order to allow partial setting so, for
instance, a dimension can rely on different models:

ldim = adg_ldim_new_full_from_model(model1, "r1", "r2", NULL);
adg_dim_set_pos_from_model(ADG_DIM(ldim), model2, "pos");