refactor: _standard_fit method made redundant. (#154)

* refactor: _standard_fit method made redundant.

* fix: xs and yz in Corr.fit promoted to arrays.

* fix: x promoted to array in _combined_fit if input is just a list.

* feat: residual_plot and qqplot now work with combined fits with
dictionary inputs.

* tests: test for combined fit resplot and qqplot added.

* docs: docstring of fits.residual_plot extended.

pyerrors/correlators.py

@@ -734,8 +734,8 @@ def fit(self, function, fitrange=None, silent=False, **kwargs):
             if len(fitrange) != 2:
                 raise Exception("fitrange has to have exactly two elements [fit_start, fit_stop]")
 
-        xs = [x for x in range(fitrange[0], fitrange[1] + 1) if not self.content[x] is None]
-        ys = [self.content[x][0] for x in range(fitrange[0], fitrange[1] + 1) if not self.content[x] is None]
+        xs = np.array([x for x in range(fitrange[0], fitrange[1] + 1) if not self.content[x] is None])
+        ys = np.array([self.content[x][0] for x in range(fitrange[0], fitrange[1] + 1) if not self.content[x] is None])
         result = least_squares(xs, ys, function, silent=silent, **kwargs)
         return result