Fix subIndex method for UGGrid

When using Hygrid::ifElse always keep in mind that some types in the functions given as "if" and "else" branch are fixed, such that compilation may fail altough the template is not instanciated.

To avoid this, those functions have one templated parameter which is instanciated with the identity for the selected branch. Passing objects through this branch will delay resolution of their type until instantiation which does only happen for the selected branch.

By passing the entity through id we use this to make subIndex compile for clang. It seems that gcc is less picky here.