Invocation object removed

When the dispatch mechanism for VTK-m worklets was originally designed, VTK-m still had to support compilers that did not implement C++11. As such, the dispatch invoke had to deal with an unknown number of arguments without support for variadic template arguments.

To help with the design, an Invocation class was made. This class is a simple struct with several template arguments containing information about the particular invocation. This includes the arguments passed to invoke (or transferred to the device), both the control and execution signatures, the domain index, the device, and multiple array types for mapping. A lot of this information is repetitious and got added to a deep stack of calls required for the dispatch. The upshot was that a lot of space was needed to represent the mangled symbols to the functions.

Now that variadic templates are supported, we can simplify the invoke quite a bit by just passing arguments directly. Consequently, the Invocation class is removed, which both simplifies the code and reduces symbol tables in compiled files.