maint: Use "fcn" as preferred abbreviation for "function" in libinterp/. * __eigs__.cc, bsxfun.cc, cellfun.cc, daspk.cc, dasrt.cc, dassl.cc, data.cc, debug.cc, error.cc, graphics.cc, graphics.in.h, gzfstream.h, ls-hdf5.cc, lsode.cc, max.cc, oct-opengl.h, quad.cc, strfns.cc, utils.cc, utils.h, variables.cc, __ode15__.cc, gzip.cc, cdef-manager.cc, ov-fcn-handle.cc, ov-java.cc, ov-usr-fcn.cc, bp-table.cc, bp-table.h, lex.h, lex.ll, oct-parse.yy, pt-eval.cc: Replace "func", "fun", "fn" in documentation and variable names with "fcn".

--- a/libinterp/corefcn/dassl.cc	Tue Apr 05 01:06:00 2022 -0400
+++ b/libinterp/corefcn/dassl.cc	Tue Apr 05 08:33:58 2022 -0700
@@ -367,11 +367,11 @@
 
   double tzero = out_times (0);
 
-  DAEFunc func (dassl_user_function);
+  DAEFunc fcn (dassl_user_function);
   if (dassl_jac.is_defined ())
-    func.set_jacobian_function (dassl_user_jacobian);
+    fcn.set_jacobian_function (dassl_user_jacobian);
 
-  DASSL dae (state, deriv, tzero, func);
+  DASSL dae (state, deriv, tzero, fcn);
 
   dae.set_options (dassl_opts);