[ntuple] minor improvements to field creation from type string
[ntuple] minor improvements to field creation from type string

olifre/root,karies/root,karies/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,karies/root,olifre/root,olifre/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,olifre/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root