Add gas monitoring like on Power Flow Card

[ElMarco99]

Hi and thanks for the fork. I would suggest to add the ability to see the gas usage like on the "vanilla" Power Flow Card.

[flixlix]

Thank you very much. This should still be possible using the individual devices and simply changing the unit

[raptile]

but it's not because of the multiply by 1000 when using m3.
if u do not set a unit type it displays they number correctly but with KW behind it.

1,234 will be displayed as 1,123kW or will become 1.234.000 when u set it to the unit m3.
And whatever custom sensor i create, it will not print 1,123 m3. :(