Fix MDAnalysis#2606: moving local function to the top (MDAnalysis#2615)

* Fixes MDAnalysis#2606
   Pytest and/or multiprocessing fails to pickle local functions. Found a workaround here:
   https://bugs.python.org/issue33884 (Basically, moving the local function to the top level.)
* the function which is being evaluated in parallel is global now
* make parallel test xfail for windows (because even this workaround does not help under windows)

testsuite/MDAnalysisTests/analysis/test_encore.py

@@ -43,6 +43,9 @@
 import MDAnalysis.analysis.encore.confdistmatrix as confdistmatrix
 
 
+def function(x):
+    return x**2
+
 class TestEncore(object):
     @pytest.fixture(scope='class')
     def ens1_template(self):
@@ -117,13 +120,9 @@ def test_triangular_matrix(self):
 inconsistent results")
 
     @pytest.mark.xfail(os.name == 'nt',
-                       strict=True,
                        reason="Not yet supported on Windows.")
     def test_parallel_calculation(self):
 
-        def function(x):
-            return x**2
-
         arguments = [tuple([i]) for i in np.arange(0,100)]
 
         parallel_calculation = encore.utils.ParallelCalculation(function=function,