BUG : raise exception in subplot if num out of range

if subplot(rows, cols, num) is called with num not in
0 < num <= rows * cols raise a ValueError

close #3166
Properly deprecate num=0 in subplot related functions
DOC : grammar/history fixes
Merge pull request #3167 from tacaswell/subplot_index_fix

BUG : raise exception in subplot if num out of range