gridinfo.hh: Use `subEntities(codim)` method instead of removed `count<codim>()` template method

The count<codim>() template method was removed from the Entity facade class in 0cfc8f31

In addition this merge requests add a simple test program to make sure the functions provided by dune/grid/common/gridinfo.hh are at least instantiated and do not fail to compile, and the code is modernized to make use of the range-based for statement.