Add libmesh_vector_at macro + Utility
Add libmesh_vector_at macro + Utility

This gives a better error message than std::vector::at(), but is still
terse enough to be convenient for bounds checking in opt mode (e.g. for
MeshInput subclasses)

capitalaslash/libmesh,libMesh/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,libMesh/libmesh,capitalaslash/libmesh,libMesh/libmesh,roystgnr/libmesh,roystgnr/libmesh,libMesh/libmesh,libMesh/libmesh,roystgnr/libmesh,capitalaslash/libmesh,capitalaslash/libmesh,capitalaslash/libmesh,dschwen/libmesh,roystgnr/libmesh,dschwen/libmesh,capitalaslash/libmesh,dschwen/libmesh,jwpeterson/libmesh,libMesh/libmesh,dschwen/libmesh,libMesh/libmesh,dschwen/libmesh,capitalaslash/libmesh,libMesh/libmesh,dschwen/libmesh,capitalaslash/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,dschwen/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,roystgnr/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,dschwen/libmesh,jwpeterson/libmesh,jwpeterson/libmesh