Introduce new class IteratorFacade and add proxy iterator support

The new CRTP-mixin class IteratorFacade is parameterized by implementation, iterator category, value_type, reference, pointer, and difference_type. Discussion of the changes:

• Making the iterator category a template parameter, allows to work with the facades class in a more uniform way. E.g. comparisons only need to be defined once for all categories.
• Being able to customize pointer allows to use the facade classes for proxy-iterators. This was not possible before, because it is not sufficient to simply exchange the reference type to support proxies.
• The design is in line with the std::iterator_interface proposal, in the sense that the template parameters have the same order to avoid surprises. Notice however, that this is not a drop-in replacement for two reasons:
  • std::iterator_interface is parameterized by the iterator concept instead of the category tag.
  • srd::iterator_interface avoids classic CRTP by deducing this.
• The old facade classes are aliases to IteratorFacade with the appropriate category tag. They also now have an additional template parameter for pointer that defaults to value_type*. Notice that the position of this template parameter is (confusingly, but for good reason) different to IteratorFacade: For the old facade classes it is appended to keep backward compatibility, for IteratorFacade it follows the order of std::iterator_interface.
• The old facade classes remain unchanged for backward compatibility reasons.

A major consequence of this change is, that it allows to use the facades for proxy-iterators, like e.g. TransformedRangeIterator. In this case you have to specify a suitable proxy-type for the pointer. In this case the new helper Dune::ProxyArrowResult can be used. This was adopted from the TransformedRangeIterator, but the name was adjusted to the std::iterator_interface proposal.

This MR also includes some cleanups to TransformedRangeIterator made possible by the new facades and preparing it for supporting free operators.