[ModelPart] support for range based loops

[philbucher]

I find it quite annoying that in order to loop the nodes one has to do:

for (auto node_it=model_part.NodesBegin(); node_it!=model_part.NodesEnd(); ++node_it) {
    (*node_it)->Id()
}

this PR adds support for range based loops:

for (auto& r_node : model_part.Nodes()) {
    r_node->Id()
    // r_node.Id() // this is done using boost::indirect_iterator in Kratos. 
    // We could implement our own iterator but not sure if it is worth it ...
}

lets discuss the implementation

[philbucher]

we decided to adapt the dereferencing iterator (i.e. the same that boost::indirect_iterator does) from the PointerHashMapSet of Kratos