New ps_apps() function to list all running applications on macOS.

New function ps_disk_io_counters() to query disk I/O counters
(#145, @michaelwalshe).

New ps_fs_info() to query information about the file system of one
or more files or directories.

New ps_wait() to start an interruptible wait on multiple processes,
with a timeout (#166).

ps_handle() now allowes a numeric (double) scalar as the pid, as long
as its value is integer.

ps_send_signal(), ps_suspend(), ps_resume(), ps_terminate(),
ps_kill(), and ps_interrupt() can now operate on multiple processes,
if passed a list of process handles.

ps_kill() and ps_kill_tree() have a new grace argument.
On Unix, if this argument is not zero, then ps_kill() first sends a
TERM signal, and waits for the processes to quit gracefully, via
ps_wait(). The processes that are still alive after the grace period
are then killed with SIGKILL.

ps_status() (and thus ps()) is now better at getting the correct
status of processes on macOS. This usually requires calling the external
ps tool. See ?ps_status() on how to opt out from the new
behavior (#31).