[Solvers] Use same naming convention for classes

include/pininvdyn/solvers/fwd.hpp

include/pininvdyn/solvers/solver-HQP-base.hpp

include/pininvdyn/solvers/solver-HQP-eiquadprog-fast.h

include/pininvdyn/solvers/solver-HQP-eiquadprog-rt.hpp

include/pininvdyn/solvers/solver-HQP-eiquadprog-rt.hxx

include/pininvdyn/solvers/solver-HQP-eiquadprog.hpp

include/pininvdyn/solvers/solver-HQP-factory.hpp

include/pininvdyn/solvers/solver-HQP-factory.hxx

src/solvers/solver-HQP-base.cpp

src/solvers/solver-HQP-eiquadprog-fast.cpp

src/solvers/solver-HQP-eiquadprog.cpp

src/solvers/solver-HQP-factory.cpp

unittest/hqp_solvers.cpp

unittest/invdyn-formulation.cpp