Fix sign of XAS

To get the sign of the XAS right (positive absorption peak) we used to look at the name of the channels.

This MR looks at the sign of the average Io and It signal to determine the correct signs.

Closes #61 (closed)

doc/changelog.rst

@@ -6,6 +6,7 @@ unreleased
 
 - **Bug fixes**
 
+    - fix :issue:`61` regarding sign of XAS in some cases :mr:`207`
 
 - **Improvements**