Back-end development on energy reporting

[Flix6x]

The task is to add a FlexMeasures CLI task (flexmeasures show total) that prints the total energy consumption of the battery in our toy tutorial.

Steps:

0. Checkout the FlexMeasures code from Github to your PC.
1. Install Docker.
2. Build the development stack (docker compose build) and run it (docker compose up), see our documentation here.
3. The command docker exec -it flexmeasures-server-1 bash -c "flexmeasures show asset --id 3" (run in separate terminal) should show the battery asset (see example)
4. Now to actual code adaptation. I want a CLI task that prints the total energy consumption of the battery's power sensor. For this, you should find the relevant place in the code where we want to define what flexmeasures show total will do (hint: we work on the CLI). Then implement the logic (for inspiration, see the code for flexmeasures show beliefs).
5. After re-building and re-running the docker compose stack, the flexmeasures show total --sensor-id 2 command (see step 3 on how to execute this command) should now show the battery's total energy consumption.

Bonus: add CLI options to set a start and duration so that the CLI command could be used to calculate a total from some starting date for some period. For example: adding --start 2022-08-01 --duration P1M to the command would give the total consumption for August.