Add ArrayHandleRuntimeVec

The new ArrayHandleRuntimeVec is a fancy ArrayHandle allows you to specify a basic array of Vecs where the number of components of the Vec are not known until runtime. (It can also optionally specify scalars.) The behavior is much like that of ArrayHandleGroupVecVariable except that its representation is much more constrained. This constrained representation allows it to be automatically converted to an ArrayHandleBasic with the proper Vec value type. This allows one part of code (such as a file reader) to create an array with any Vec size, and then that array can be fed to an algorithm that expects an ArrayHandleBasic of a certain value type.