Copy data from one file / back-end to another

At the moment it is not easily possible to copy content from one file or back-end to another. One solution to this problem would be to add create methods that accept another instance of the entity to create:

``` c++
DataArray da_new = block.createDataArray(da_old);
```

Another possibility is to overwrite data of an existing entity with data from another one:

``` c++
DataArray da = ....;
DataArray da_other = ...;
da.apply(da_other)
```

It has to be specified to what extent those method also copy data of their respective child entities:

``` c++
// does this copy also all Source, DataArray and other entities?
// What about attached Sections?
Block b_new = file.createBlock(b_old);
```