Propagate to non-KDL classes the improved definition of URDF "fake link" introduced in https://github.com/robotology/idyntree/pull/246

[traversaro]

PR 246 : https://github.com/robotology/idyntree/pull/246

Allow to have two fake links (i.e. "additional frames") connected to one another through fixed joints. This happens on the Walkman model.

This reduces to modify the removeFakeLinks function:

• https://github.com/robotology/idyntree/blob/2470a71cee49ee3a611195e36b8dc447eca9f154/src/model/include/iDynTree/Model/ModelTransformers.h#L47
• https://github.com/robotology/idyntree/blob/2470a71cee49ee3a611195e36b8dc447eca9f154/src/model/src/ModelTransformers.cpp#L56
• http://wiki.icub.org/codyco/dox/html/idyntree/html/namespaceiDynTree.html#aab77480c8f9d5add110768cf75226508

Note that the behavior implemented in https://github.com/robotology/idyntree/pull/246 can be easily simulated by two calls to the current version of removeFakeLinks , the first that remove the "leaf" fake links and the second that remove any parent. This can be easily generalized to just iterate the process until the numnber of links is contact before and after the call to removeFakeLinks.

[traversaro]

See also : https://github.com/robotology/idyntree/pull/246#discussion_r88617972 .