In scan constructor, interpret limits as scan points, not as bin edges (#423)

This resolves #423 (closed).

Base/Axis/FixedBinAxis.cpp

@@ -127,3 +127,14 @@ bool FixedBinAxis::equals(const IAxis& other) const
     }
     return false;
 }
+
+//... In global namespace.
+
+FixedBinAxis FixedScanAxis(const std::string& name, size_t nbins, double scan_start,
+                           double scan_end)
+{
+    if (nbins < 2)
+        throw std::runtime_error("Cannot create scan axis with less than two points");
+    const double delta = (scan_end - scan_start) / (nbins - 1);
+    return FixedBinAxis(name, nbins, scan_start - delta / 2, scan_end + delta / 2);
+}