Implement "Device Definitions" with user defined ModBus registers

[luistripa]

The device definitions are a way for users to create custom mappings of ModBus registers in order to support devices which have different registers from SMA Inverters (or even among different versions of SMA Inverters)

For example, say we have 2 inverters, A and B, which have different register addresses regarding power production. With the device definitions system, we would create a "Power Production" state common to both devices, however, for each device the actual requested register would be different.