Provide retail pricing dummy sensor by applying a formula over PUN prices

[Ocramius]

Ref: #6
Ref: #10

As explained by @ virtualdj, retail prices depend on the current contract with the electricity retailer, and therefore need to be computed with a template sensor.

This patch provides an example, although it is unclear where this would be positioned in configuration.yml

[Ocramius]

A bit unclear to me under which key I'd configure such a template value.

Also: is this configurable by UI, or would we point people at adding this to their configuration.yml?

[Ocramius]

Meanwhile, my local formula, if I understood this correctly:

template:
  - sensor:
      # see https://github.com/virtualdj/pun_sensor/pull/11
      - unique_id: current_retail_electricity_price
        name: 'Current retail electricity price'
        icon: mdi:currency-eur
        unit_of_measurement: '€/kWh'
        # 1.102 = 10.2% spread
        # 1.038 = 3.8% MT (not sure what this is)
        # 0.073 = delivery fees
        # 0.042 = distribution/measurement fees
        state: >
          {{ (1.102 * 1.038 * (states('sensor.pun_prezzo_fascia_corrente')|float(0) + 0.073 + 0.042))|round(3) }}

Probably mixed up the formula a bit, so please do feel free to correct me, so I both get it right for myself and for others here :D

EDIT: in fact, I think it's like this

template:
  - sensor:
      # see https://github.com/virtualdj/pun_sensor/pull/11
      - unique_id: current_retail_electricity_price
        name: 'Current retail electricity price'
        icon: mdi:currency-eur
        unit_of_measurement: '€/kWh'
        # 1.102 = 10.2% spread
        # 1.038 = 3.8% MT (not sure what this is)
        # 0.073 = delivery fees
        # 0.042 = distribution/measurement fees
        state: >
-          {{ (1.102 * 1.038 * (states('sensor.pun_prezzo_fascia_corrente')|float(0) + 0.073 + 0.042))|round(3) }}
+          {{ (states('sensor.pun_prezzo_fascia_corrente')|float(0) * 1.102 * 1.038 + 0.073 + 0.042)|round(3) }}

[virtualdj]

To be honest, I don't remember at the moment where I found that formula... and I'm not sure it's actually perfect.
For example grid losses and dispatchment costs are based on the kWh (so they must be multiplied) but there are also monthly costs that are not included here.

Anyway, everything it's just an indication because you won't get the correct result until near the end of the month.