Introduce range wrapper for OneToManyRelations and VectorMembers

[tmadlener]

BEGINRELEASENOTES

• Introduce range wrapper for OneToManyRelations and VectorMembers to allow range-based for loops

ENDRELEASENOTES

Requires #106
Fixes #61

Currently to loop over related objects one has to do something like this

ExampleMC mcp;
// fill with some daughters
for (auto it = mcp.daughters_begin(); it != mcp.daughters_end(); ++it) {
  // ...
}

A podio::RelationRange is introduced here. It provides begin and end and allows for range-based for loops:

ExampleMC mcp;
// fill with some daughters
for (const auto& p : mcp.daughters()) {
  // ...
}

The tests that come with this break, because #105 needs to be fixed first (by #106)

[tmadlener]

Fixes #61