Change the VTKM_CONT_EXPORT to VTKM_CONT. (Likewise for EXEC and
EXEC_CONT.) Remove the inline from these macros so that they can be
applied to everything, including implementations in a library.
Because inline is not declared in these modifies, you have to add the
keyword to functions and methods where the implementation is not inlined
in the class.
Template the splatter algorithm over Kernel type and use abstract kernel
interface to fetch the kernel value.
Add Gaussian, Spline3rdOrder kernel classes templated over dimension.
Other kernels can/will be added in future.
Kernel classes are defined such that the integral of the volume is unity
as is the convention in (for example) SPH simulations.
