Assign a value to system

[samuelolteanu]

Great work. As the title says, I'm unable to display an entity value in system object. I suspect this is intentional, it is calculated, but in my case, there are some loses (inverter has some power usage) and it's just more precise for me to display the measurement of its ouput. I also don't export energy and it would be more aestetically pleasing to show a positive value in system. Awesome job on the extra entities, I think i have a good replacement for tesla-style-solar-power-card by reptilex.