Block: enable links across blocks

[gicmo]

[jgrewe]

Idea of this enhancement is to allow links between entities that exist in different blocks in the same file. It would be very convenient to use this for example to separate raw data from derived data.

At the moment I see one major drawback; names of entities must be unique across in the whole file (and, if we at some point allow linking to external files, even among those...). Having e.g. two recording sessions with the same using names in the same file would not be possible. In the recording context this would be annoying and we would end with names like 'data_01' etc.