rcpparma_hello_world() does not return a value, but displays a
message to the console.
rcpparma_outerproduct() returns a numeric matrix computed as the
outer (vector) product of x.
rcpparma_innerproduct() returns a double computer as the inner
(vector) product of x.
rcpparma_bothproducts() returns a list with both the outer and
inner products.