Add common superclass to VariantArrayHandleBase

Many of the operations of VariantArrayHandleBase are not dependent on the TypeList parameter of the class. Still others can operate just as well by providing a type list to a method. Thus, it is convenient to create a superclass that is not templated. That allows us to pass around a VariantArrayHandle when the type list does not matter.

This superclass is called VariantArrayHandleCommon because "base" was already taken.