Mimic stl and boost constructors in our allocators
Mimic stl and boost constructors in our allocators

Neither boost  nor stl (at least not in libstdc++ implementation) use
`explicit` on their copy constructors. And more importantly for me to be
able to build on my machine I need to make the copy constructor
templated to allow copy construction from `(Fast)PoolAllocators` with
different `T`. Refs #2936

libMesh/libmesh,BalticPinguin/libmesh,dschwen/libmesh,libMesh/libmesh,libMesh/libmesh,dschwen/libmesh,libMesh/libmesh,libMesh/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,libMesh/libmesh,BalticPinguin/libmesh,dschwen/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,dschwen/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,dschwen/libmesh,dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,dschwen/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,libMesh/libmesh