Name as Id

Use the entity name instead of the id as group name in HDF5. Implemented so far only on DataArrays.

Forks from branch "let_refs" which took care of using references in the frontend wherever possible.
If you put the check that now occurs inside the lambda here, you could save some group reading in cases where the group was already found:

For example:
```c++
if(group.hasAttr(attribute)) {
    string tmp;
    group.getAttr(attribute, tmp);
    if (tmp == value) {
        ret = group;
        break;
    }
}
```

And btw: space between `if` and `(` and newline after `if ( ... )`