integer types in interface cleaned up

The interface of entities is cleaned up, i.e. everything follows the grid-interface:

• corners() returns int (see Dune::Geometry)
• level() returns int (see Dune::Entity)
• maxLevel() returns int (see Dune::Grid)
• size() returns int (see Dune::Grid, Dune::GridView, Dune::ReferenceElement,...)

This eliminates the "warning: comparison between signed and unsigned integer expressions"