iterator_traits does not work when data_type is convertible to DataWeight

[Peter]

iterator_traits is specialized for case when value_type is DataWeight so iterator is weighted. Iterators that have value_type convertible to DataWeight? should be treated likewise.

[Peter]

(In [2159]) fixes #586 and refs #587 extend test of Averager and AveragerWeighted?. Document requirement in iterators.

weighted_iterator_traits<T>::type now returns weighted if T is convertible to DataWeight? (refs #604).

[Peter]

(In [2164]) improved docs. refs #604

[Peter]

(In [2165]) complete r2164. refs #604

[Peter]

The idea is to specialize the template on Iterator::reference having function

double& data(void)

rather than specialize on when Iterator::reference is DataWeight.

Seems like the technique explained ​here could be used to accomplish this.

[Peter]

(In [2166]) reimplemented iterator_traits. For unweighted iterators it works as before. For weighted iterators the default implementation is now extended and works as long as *Iter has functions data(void) and weight(void). If these functions return double the corresponding [data|weight]_reference is set to double& and otherwise it is set to const double. This implies that there is no need to have specializations for StrideIterator? or WeightedIterator? so these are removed.

refs #604