Have more CamelCase methods

The Dune style guidelines require camel-case method names. However, some dune-pdelab methods use lower_case_with_underscores. This inconsistency is difficult to justify, and therefore this patch renames several such methods in constraints.hh and adaptivity.hh to camel-case naming. To preserve backward- compatibility, methods with the old names are reintroduced, and simply forward the renamed-implementations. These old-name methods are not deprecated, to make this patch as uncontroversial as possible.