Support resid_handle in bg fitting of pwader class

And add test

+sw_tests/+unit_tests/unittest_sw_fitpowder.m

@@ -14,6 +14,10 @@
         default_modQ_cens_1d = 3.55:0.1:5.45; % integrate over nQ pts
     end
 
+    properties (TestParameter)
+        fit_params = {{}, {'resid_handle', true}};
+    end
+
     methods (TestClassSetup)
         function setup_spinw_obj_and_expected_result(testCase)
             % setup spinw object
@@ -321,13 +325,13 @@ function test_estimate_constant_background(testCase)
             testCase.verify_results(out, expected_fitpow);
         end
 
-        function test_fit_background(testCase)
+        function test_fit_background(testCase, fit_params)
             out = sw_fitpowder(testCase.swobj, testCase.data_2d, ...
                                testCase.fit_func, testCase.j1);
             out.y(1) = 10;  % higher so other bins are background
             out.fix_bg_parameters(1:2); % fix slopes of background to 0
             out.set_bg_parameters(3, 1.5); % initial guess
-            out.fit_background()
+            out.fit_background(fit_params{:})
             expected_fitpow = testCase.default_fitpow;
             expected_fitpow.y(1) = 10;
             expected_fitpow.ibg = [3;6;2;5;4];

swfiles/sw_fitpowder.m

@@ -549,7 +549,13 @@ function fit_background(obj, varargin)
                 error('spinw:sw_fitpowder:fit_background', ...
                       'Not enough points to fit the function.');
             end
-            [bg_params, ~, stat] = ndbase.simplex([], @obj.calc_cost_func_of_background, ...
+            if any(cellfun(@(elem) elem =="resid_handle", varargin(1:2:end)))
+                fobj = @obj.calc_residuals_of_background; % minimise using residual array
+            else
+                fobj = @obj.calc_cost_func_of_background; % minimise scalar
+            end
+
+            [bg_params, ~, stat] = ndbase.simplex([], fobj, ...
                                                   obj.params(ibg_par)', ...
                                                  'lb', obj.bounds(ibg_par,1)', ...
                                                  'ub', obj.bounds(ibg_par,2)', ...