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.
There are various reasons why you might want to execute something but
not have a specific device to execute on. To mange this, add a general
function that will try a list of devices in order and attempt to run on
them in order.