Update cell_subsystem(s) keys

[rowlesmr]

As discussed in #54

• Added _cell_subsystem.structure_id as linked composite key
• Added _cell_susbsystems.id as unique Set key
• Rephrased definition of _cell_subsystem.code to remove reference to data blocks
• Added _cell_subsystem.subsystems_id as linked non-key data name

This is my interpretation as to how subsystems work nowadays.

[jamesrhester]

That seems to accord with @gmadaria 's description of how it is conceptualised.