Add converters as links and enable DC lines

[davide-f]

Describe the feature you'd like to see

HVDC lines and converters are not properly modelled yet. PR #348 will focus on the topology side to make sure that we are able to properly identify DC lines and have a complete network, yet all lines,buses and (fake-)converters will be treated as AC lines.
This issue shall focus on moving beyond that and model converters as links and HVDC lines as dc lines.

Note: In PyPSA-Eur DC lines are also modelled as links, though it may be better to model them as dc lines if it will be possible. With respect to the application to Africa, given the current status of the lines, no differences should occur.

[davide-f]

As also discussed with @fneum , treating DC lines as "Line" PyPSA component, instead of a "Link" is feasible and would need limited changes with respect to the pypsa-eur implementation.
So when this issue will be addressed, the goal is:

• Enable converters modelled as "Link" PyPSA components. PyPSA-Eur will be the major example to take inspiration from
• DC lines shall be kept stored in the lines.csv, but with "dc" flag being true
• DC lines will be modelled as DC lines, instead of links, as done in PyPSA-Eur

PyPSA-Eur will serve as a good basis, yet debugging is needed

[davide-f]

The recommendation to address this is to work step by step and revise in order:

• clean_osm_network
• build_osm_network
• base_network

In these, we should make sure to have the converters input/output file, alike the other components.

In clean_osm_network, the focus should be in revising the get_converters function and create as output a dataframe having the structure that adheres with the type "Link" of PyPSA.

In build_osm_network the focus shall be into adapting the new converter approach and make sure the matching works fine and the network is complete.

In base_network, finally the converters shall be added as "Link" component.

In the scripts, the fix of setting everything as AC shall drop in the end, if possible.