Add support for dynamic order to LagrangeDGPreBasis

This add support for providing the polynomial order dynamically to LagrangeDGPreBasis analogously to ´LagrangePreBasis`. Additionally this implements some minor improvements/changes:

• Allow to select range type using a template parameter.
• Base implementation on LeafPreBasisMapperMixIn to drop a bit of boilerplate code.
• Improve documentation.
• Deprecate the dofsPer(...) static member variable that only make sense for compile time order.