maint: Deprecate is_object and replace with isobject. * ov.h (is_object): Use OCTAVE_DEPRECATED macro around function. * ov.h (isobject): New function. * bsxfun.cc, cellfun.cc, data.cc, ls-mat5.cc, oct-stream.cc, symtab.h, variables.cc, ov-base.h, ov-class.cc, ov-class.h, ov-classdef.h, ov-oncleanup.h, ov-struct.cc, ov.cc, ov.h, op-class.cc, pt-arg-list.cc, pt-eval.cc, pt-tm-const.cc: Replace instances of is_object with isobject.

--- a/libinterp/corefcn/cellfun.cc	Tue Jun 13 10:20:55 2017 -0700
+++ b/libinterp/corefcn/cellfun.cc	Tue Jun 13 11:25:31 2017 -0700
@@ -1822,7 +1822,7 @@
             retval = do_num2cell (array.array_value (), dimv);
         }
     }
-  else if (array.is_object ())
+  else if (array.isobject ())
     retval = do_object2cell (array, dimv);
   else if (array.isstruct ())
     retval = do_num2cell (array.map_value (), dimv);