[C++][Bug] Avoid confusion between joint and body methods in se3::Model.

Try to avoid travis killed builds (i.e crashes due to building environment running out of ram)

[travis] Build_type is now MinSizeRel to optimize the comsumption of RAM during process in order to avoid crashes during travis builds. Display memory during build to track RAM information. Reduce number of dependencies

Topic/binding joints

  * se3::Model::getBodyId was used to retrieve joint id in parsers.
  * Implement new methods to retrieve joint ids.
  * Fix body methods.

[C++][Bug] Enforce One Definition Rule in template instantiations.

  * Add inline specifier where needed.
  * This is done to avoid multiple definition when using the pinocchio
    library as a dependency.

[travis] Reorganized travis build to perform 4 tests : gcc Debug and Release , clang Debug and Release. For now, only one test is done to ensure that builds that failed were due to travis build environment running out of RAM

[Python] Binding the different joint models. The exposure is done through the variant. In joints-models, you can specialise the binding for any jointModel you want.

[C++][Bug fixed] Changed from assignation to initialization...

[hotfix] Changed from assignation to initialization for a matrix in symmetric3 that causes computations errors with ubuntu 14.04 in release mode.

Topic/joint dense

[C++]{Minor] Changed the name of the method to access the segment of a given vector. The length of the returned segment depends on the dof of the joint(size NQ or NV)

[C++][Major] Reorganized the methods to acces segments of fixed size (one for NQ, one for NV) from joints indexes

[C++][Major] Added JointModelDense and JointDataDense. Both templated by the dimension of configuration space and tangential space. Each joint can be converted to a Dense one

[Fix Bug] Check norm of the vector to exponentiate

topic/crtp - Convert all current structures to the CRTP template

[Major][C++] Renamed macro for spatial traits : 2 cases -> use in templated/not templated classes to avoid polluting warnings. Following these naming could avoid multiple definitions of macro SE3_JOINT_TYPEDEF

[C++][Major] MotionRevolute is now CRTP. Moved out of struct JointRevolute to solve namespace problems

Conflicts:

	benchmark/timings.cpp

[Major] BiasZero is now a MotionBase (CRTP), implemented in jointrevolute. Switch in other joints to come

[Major] spatial class SE3 is now CRTP. Move traits class in spatial/fwd.h as it is now used in spatial classes and joint-base