Deprecate the meter_registers.nature field

[damonrand]

This is redundant because the operator applies the circuits.type and this is linked through the circuit_registers join to the meter register.

A particular case arose as follows:
-> A meter register had a heat circuit connected.
-> That register was then rewired to a power circuit.
-> No changes were made to the meter or registers.
-> We were able to model this with circuit_registers start/end times.
-> Then all querying/reporting is going through circuits.
-> So the nature of the register changed over time.