`vtkm::source::Source` now uses virtual inheritance for
the `Execute` method. This will allow us to enforce that
all vtkm::source have a consistent calling convention.

To make the code inside filters and sources more similar the
base `vtkm::source::Source` class has a `vtkm::cont::Invoker`
as a member variable. This will allow sources to use the
`this->Invoke()` pattern that filters use to launch worklets.