TopologyGenerators.generateRing() does not position the nodes correctly

[acasteigts]

When creating nodes with TopologyGenerators.generateRing(), the nodes are not well placed correctly, as witnessed by the attached picture.

[remikey]

Cause of the problem: TopologyGenerators.generateRing() had incoherent positioning and dimensioning values.

Fix: A new method, which centers the shape in the Topology and sizes it to secure a margin.
Bonus: for consistency sake, TopologyGenerators.generateKN() now also uses that method.

[remikey]

@acasteigts , please have a look at branch fix/111-topologygenerators-generateRing-positioning.

Note: It is not based on develop, but on the branch which fixes #108 .

[remikey]

merged into develop.