Use std::vector in t_state

Use std::vector in t_state

Changed x, v and cg_p pointers in t_state to PaddedRVecVector.
PaddedRVecVector is currently std::vector<RVec> and is always
sized with one element extra for 4-wide SIMD (gather) loads.
But this will be replaced by a proper class.
t_state is now freed by the default destructor with some issues:
* ekinstate_t is fully converted to std::vector in a child change
* state struct of special algorithms still need conversion

Changes to t_state propagate over large parts of the core code.
This change extracts rvec pointers at intermediate levels to
make this single change manageable.
The state in EM also needed to be updated, which led to a lot
of changes in minimize.cpp, especially to do_lbfgs() which was not
yet converted to use em_state_t.

Change-Id: Ib6ead1f13741ead8dbeeddd118ecdd4bf1cc6584