Mark ArrayHandle constructors taking buffers, as explicit

This resolves a compiler ambiguity I hit during development. In my case, I created an ArrayHandleDecorator with one of the arrays being an ArrayHandleTransform. The ambiguity occurs in function DecoratorStorageTraits<...>::BuffersToArray[1], here an ArrayHandleTransform is constructed using buffers (std::vector<vtkm::cont::internal::Buffer>)

This constructor is not defined for ArrayHandleTransform, but it's defined for its superclass (vtkm::cont::ArrayHandle)[2]. ArrayHandleTransform does have a non-explicit constructor that takes the array to be transformed and the transform-functor as parameters, where the later has a default value[3]. This produces the following ambiguous options for the compiler:

1. Create an instance of the ArrayHandle that is to be transformed, using the buffers[2], and call the ArrayHandleTransform constructor with this array with the defaulted functor parameter [3].
2. Create the superclass instance using the buffer[2] and call the ArrayHandleTransform constructor that takes the superclass[4].

In this situation, option 2 is the correct choice.

The ambiguity is resolved by marking the constructors that take buffers as explicit. These constructors are also added for the derived classess via the VTK_M_ARRAY_HANDLE_SUBCLASS_IMPL macro.

1. https://gitlab.kitware.com/vtk/vtk-m/-/blob/ddada34223b81e622292edc14a6ed57b6834348c/vtkm/cont/ArrayHandleDecorator.h#L363
2. https://gitlab.kitware.com/vtk/vtk-m/-/blob/ddada34223b81e622292edc14a6ed57b6834348c/vtkm/cont/ArrayHandle.h#L340
3. https://gitlab.kitware.com/vtk/vtk-m/-/blob/ddada34223b81e622292edc14a6ed57b6834348c/vtkm/cont/ArrayHandleTransform.h#L456
4. https://gitlab.kitware.com/vtk/vtk-m/-/blob/ddada34223b81e622292edc14a6ed57b6834348c/vtkm/cont/ArrayHandle.h#L182