Rename dcline to simple_dc_link

[jkisse]

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Issue

The two components dcline and line_dc can easily be confused and the documentation is not very precise in pointing out the difference between the two. I propose to rename "dcline" to "simple (HV)DC-link", as the electrical model apparently assumes converter stations as well.
The documentation and docstrings should be expanded and also mentioned, that dcline and line_dc are two completly different things and not intended to be used together.

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[vogt31337]

Really quite confusing, but I don't understand what a line_dc is? Where is it used and for which use case?
The dcline is modeled using generators but from the docs a line_dc seems to be a normal line? Is this for dc loadflows?!

[jkisse]

I think line_dc is intended to be used with VSCs . It wouldn't make sense to use these converters with the dcline.