Expose states collection class

Introduce States class for extrapolation of the entire collection of states. Needed for Rec!2580 (merged) Moore!1160 (merged).

Event/TrackEvent/include/Event/States.h

+/*****************************************************************************\
+* (c) Copyright 2000-2021 CERN for the benefit of the LHCb Collaboration      *
+*                                                                             *
+* This software is distributed under the terms of the GNU General Public      *
+* Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   *
+*                                                                             *
+* In applying this licence, CERN does not waive the privileges and immunities *
+* granted to it by virtue of its status as an Intergovernmental Organization  *
+* or submit itself to any jurisdiction.                                       *
+\*****************************************************************************/
+
+#pragma once
+
+#include "Event/SOACollection.h"
+#include "Kernel/EventLocalAllocator.h"
+#include "LHCbMath/MatVec.h"
+#include "LHCbMath/SIMDWrapper.h"
+#include "PrTracksTag.h"
+
+/** @class States States.h
+ *  SoA implementation of states class
+ */
+
+namespace LHCb::Event::v3 {
+
+  namespace StatesTag {
+
+    struct State : Event::state_field {};
+    struct Covariance : Event::state_cov_field {};
+    struct Success : Event::int_field {};
+
+    template <typename T>
+    using states_t = Event::SOACollection<T, State, Covariance, Success>;
+  } // namespace StatesTag
+
+  struct States : StatesTag::states_t<States> {
+    using base_t = typename StatesTag::states_t<States>;
+    using base_t::base_t;
+  };
+
+} // namespace LHCb::Event::v3