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.