Check whether a future is resolved or not
A logical of the same length and dimensions as
Each element is TRUE unless the corresponding element is a
non-resolved future in case it is FALSE.
This method needs to be implemented by the class that implement
the Future API. The implementation should return either TRUE or FALSE
and must never throw an error (except for FutureError:s which indicate
significant, often unrecoverable infrastructure problems).
It should also be possible to use the method for polling the
future until it is resolved (without having to wait infinitely long),
while (!resolved(future)) Sys.sleep(5).