STY Cleaner ylabel for sensitivity plotter

No longer uses conditionals inside format statement, instead
uses " ".join([list, of, parts])

Remove an additional re-setting of the xlabel that was missed
in GH #405

Author: drewejohnson

serpentTools/parsers/sensitivity.py

@@ -415,13 +415,16 @@ def plot(self, resp, zai=None, pert=None, mat=None, mevscale=False,
         if xlabel is None:
             xlabel = "Energy [MeV]" if mevscale else "Energy [eV]"
 
-        xlabel = 'Energy [eV]' if xlabel is None else xlabel
-        ylabel = ylabel if ylabel is not None else (
-            'Sensitivity {} {}'.format(
-                'per unit lethargy' if normalize else '',
-                r'$\pm{}\sigma$'.format(sigma) if sigma else ''))
+        if ylabel is None:
+            parts = ["Sensitivity"]
+            if normalize:
+                parts.append("per unit lethargy")
+            if sigma:
+                parts.append(r"$\pm{}\sigma$".format(sigma))
+            ylabel = " ".join(parts)
+
         ax = formatPlot(ax, loglog=loglog, logx=logx, logy=logy, legendcols=ncol,
-                        legend=legend, xlabel=xlabel, ylabel=ylabel.strip())
+                        legend=legend, xlabel=xlabel, ylabel=ylabel)
         return ax
 
     def _gather_matlab(self, reconvert):