could someone review patches on bug #48690 (statistics functions with empty inputs)

[Nicholas Jankowski]

http://savannah.gnu.org/bugs/?48690

Originally thought these compatibility fixes could fit in with 4.2 but just realized I left it sit for 2 months.

code now produces matlab compatible output for all varieties of empty inputs for std, var, mean, mode, cov.  (if there's any major problem with them, it's that I left in all the tests I used to figure out compatibility)

nick j