Make collection iterators fulfill LegacyInputIterator and input_iterator concept

[m-fila]

BEGINRELEASENOTES

• Added missing operations and type aliases so collection iterators fulfill LegacyInputIterator and std::input_iterator concept

ENDRELEASENOTES

Fixes: #150, fixes #272
Conflicts: #273

Implement missing features so CollectionIterators and MutableCollectionIterators fulfill the requirements imposed on the C++ iterators (#598). It's expected than not all requirements can be fulfilled as some are in direct conflict with podio design:

• LegacyForwardIterator: dereference should return a reference, but in podio iterators are 'proxy iterators' returning user layer objects as a proxy for reference
• LegacyForwardItertor: dereferencing two iterators that are equal should give two references bound to the same object, but in podio the returned 'proxies' are different objects
• std::forward_iterator: pointers and references obtained from an iterator should be valid as range the range is valid, but in podio the iterators behave like 'stashing iterators' -> pointers point to iterator member variable limiting their validity to the lifetime of the iterator.

As a consequence the iterators can the podio iterators can be at most LegacyInputIterator and std::input_iterator. This PR implements missing features to achieve this.

List of changes:

Named requirements:

• LegacyIterator:
  • copy constructor and assignment
  • type aliases
• LegacyInputIterator:
  • postfix increment
• LegacForwardIterator:
  • value-initialization
  • dereference return reference not proxy
  • dereferencing equal iterators returns references to the same object
• OutputIterator:
  • assignment should modify collection
• iterator_category is std::input_iterator_tag
• check selected algorithms operating on LegacyInputIterators

Concepts:

• semantics checks for concepts
• std::input_or_output_iterator
  • same as LegacyIterator but copyability not required
• std::input_iterator
  • std::indirectly_readable and std::readable (can dereference both iterator and const iterator)
• std::forward_iterator
  • syntax: constrains (except iterator_concept to be derived fromstd::forward_iterator_tag)
  • semantic: pointers obtained from iterator should remain valid as long as range is valid (no-stashing)
• iterator_concept is std::input_iterator_tag
• check selected range algorithms constrained to std::input_iterator

[tmadlener]

I know this is not fully done yet, but I think it would be nice to also document which (classes of) algorithms will be enabled by this (if any).