Various Modbus-related improvements

[Jalle19]

Allows reading e.g. these particular data points from Victron VenusOS:

  - name: VE.Bus AC-In L1
    type: circuit
    group: ESS
    sensor:
      type: modbus
      modbus:
        address: 10.112.4.250
        port: 502
        unit: 227
        register: h@12/int16
      filters:
        scale: 0.1
  - name: VE.Bus AC-In L2
    type: circuit
    group: ESS
    sensor:
      type: modbus
      modbus:
        address: 10.112.4.250
        port: 502
        unit: 227
        register: h@13/int16
      filters:
        scale: 0.1
  - name: VE.Bus AC-In L3
    type: circuit
    group: ESS
    sensor:
      type: modbus
      modbus:
        address: 10.112.4.250
        port: 502
        unit: 227
        register: h@14/int16
      filters:
        scale: 0.1
  - name: VE.Bus AC-In total
    type: circuit
    sensor:
      type: virtual
      virtual:
        children:
          - VE.Bus AC-In L1
          - VE.Bus AC-In L2
          - VE.Bus AC-In L3