maint: Deprecate is_map and replace with isstruct. * ov.h (is_map): Use OCTAVE_DEPRECATED macro around function. * ov.h (isstruct): New function. * structdemo.cc, __luinc__.cc, __magick_read__.cc, cellfun.cc, data.cc, debug.cc, error.cc, graphics.cc, load-save.cc, ls-mat5.cc, mex.cc, oct-lvalue.h, oct-stream.cc, variables.cc, __eigs__.cc, __ode15__.cc, ov-base.h, ov-class.cc, ov-class.h, ov-classdef.h, ov-java.h, ov-struct.cc, ov-struct.h, ov-usr-fcn.cc, ov.h, pt-eval.cc, pt-tm-const.cc: Replace instances of is_map with isstruct.

--- a/libinterp/corefcn/cellfun.cc	Tue Jun 13 08:24:42 2017 -0700
+++ b/libinterp/corefcn/cellfun.cc	Tue Jun 13 09:20:08 2017 -0700
@@ -1824,7 +1824,7 @@
     }
   else if (array.is_object ())
     retval = do_object2cell (array, dimv);
-  else if (array.is_map ())
+  else if (array.isstruct ())
     retval = do_num2cell (array.map_value (), dimv);
   else if (array.iscell ())
     retval = do_num2cell (array.cell_value (), dimv);