Add directed association relationship, and thicken the association arrow line

[joseluis]

The association relationhip can be undirected by default, but also directed. As seen in https://pubs.opengroup.org/architecture/archimate3-doc/chap05.html#_Toc10045321

The directed version it should present a half-arrowhead, but that one isn't available in plantuml, so the next best solution is to mantain consistency with the rest of the dependency relationships and use the same arrowhead as them.

And to avoid confusion with the serving relationship my proposal is to thick the body of the association relationship. As seen in the following diagram (which already has the corrections from PR #19)