Add edm::VecArray
Use edm::VecArray in ClusterData and ClusterShapeHitFilter

To reduce the amount of dynamic memory allocations.
Optimize SiPixelClusterShapeCache for time and space

Based on tests on standalone test program.
Optimize SiPixelClusterShapeCache for space

There is a timing hit from the bitfields, but hopefully it gets hidden
by semi-random access and other overheads in the calling code.