Allow the user to select if reciprocated links appear on a single or separate lines

[oxy86]

Up to version 2.3, SocNetV displayed reciprocated directed edges between two actors A and B in two separate lines, one for the A -> B tie and another for the A <- B tie. That was problematic in terms of memory consumption (double edge graphic items) on very large directed nets. It also made the network visualization less attractive as the user was seeing double lines between nodes.

SocNetV version 2.4 changed that by always displaying the reciprocated ties between actors in a single line with two arrows.

However, some users might still need to display reciprocated links in separate lines for their purposes.

Therefore, SocNetV should have an option in Settings allowing the user to select how reciprocated links should appear (single line or separate lines).