add new color scheme for more distinguishing colors

[fneum]

As discussed in PyPSA/pypsa-eur#591, there were many overlapping colours for the technologies such that some were not well distinguishable in the summary plots.

This is the first step to change that. But I am not yet happy with all the colours. Happy to gather your thoughts!

[lisazeyen]

I would suggest changing the colors of "resistive heaters" and "power-to-heat" to something similar as the other power-to-heat technologies, e.g.
"power-to-heat":"#4f940c"
"resistive heaters": "#d8f9b8"
I haven't tested though how similar the colors are with the battery colors.