add k-vec/ISODISTORT tutorial, with minor fix to catch uninstalled sympy

Author: briantoby

GSASII/GSASIIpwdGUI.py

@@ -5206,6 +5206,13 @@ def OnISODISTORT_kvec(event):
         '''Search for k-vector using the ISODISTORT web service.
         Developed by Yuanpeng Zhang with help from Branton Campbell.
         '''
+        if symbols is None:
+            G2G.G2MessageBox(G2frame,
+                    'Unable to perform ISODISTORT kvec search without the sympy module. Use Help/Add packages... to address','No sympy')
+            return
+        # symbolic "variables" needed below for routine to_fraction_strs()
+        a, b, g = symbols('a b g')
+
         def _showWebPage(event):
             'Show a web page when the user presses the "show" button'
             import tempfile
@@ -5259,8 +5266,6 @@ def grab_all_kvecs(out_html):
             else:
                 return None
 
-        a, b, g = symbols('a b g')
-
         def frac_str(value, max_den=1000):
             # Try to get a rational exactly; if huge, fall back to bounded approximation
             try:
@@ -5323,10 +5328,6 @@ def setup_kvec_input(k_vec, k_vec_dict, isocif_cif):
             return data_update
 
         # start of OnISODISTORT_kvec computations
-        if symbols is None:
-            G2G.G2MessageBox(G2frame,
-                    'Unable to perform ISODISTORT kvec search without the sympy module. Use Help/Add packages... to address','No sympy')
-            return
         try:
             import requests
         except:

GSASII/tutorialIndex.py

@@ -95,11 +95,14 @@
     ['Magnetic-V', 'Magnetic-V.htm',"Magnetic Structure Analysis-V",
      '''Analysis of a complex Type IV antiferromagnet with two propagation vectorse using Bilbao k-SUBGROUPSMAG from TOF neutron powder data'''],
 
-    ['k_vec_tutorial', 'k_vec_tutorial.html', 'k-vector searching in GSAS-II #1',
+    ['k_vec_tutorial', 'k_vec_tutorial.html', 'k-vector searching in GSAS-II #1 (all-zero vector)',
     '''Search neutron diffraction data of Er2Ge2O7 for a all-zero magnetic k-vector'''],
 
-    ['k_vec_tutorial_non_zero', 'k_vec_tutorial_non_zero.html', 'k-vector searching in GSAS-II #2',
+    ['k_vec_tutorial_non_zero', 'k_vec_tutorial_non_zero.html', 'k-vector searching in GSAS-II #2 (non-zero vector)',
     '''Search neutron diffraction data used in Magnetic Structure Analysis-III for a non-zero magnetic k-vector'''],
+    
+    ['k_vec_isodistort', 'k_vec_isodistort.html', 'Use of  ISODISTORT with a k-vector found by GSAS-II',
+    '''Use ISODISTORT to find isotropic subgroups that are compatible with a magnetic k-vector'''],
 
     #['ExampleDir', 'ExamplePage.html', 'Example Tutorial Title', '''Example descriptive text'''],