Conversions between quaternion and Euler

include/sot/talos_balance/euler-to-quat.hh

include/sot/talos_balance/quat-to-euler.hh

src/CMakeLists.txt

src/euler-to-quat.cpp

src/quat-to-euler.cpp

unittest/python/test_eulerquat.py

unittest/python/test_simple_zmp_estimator.py