release: Update package.xml version to 1.6.1

package.xml

 <?xml version="1.0"?>
 <package format="3">
   <name>tsid</name>
-  <version>1.6.0</version>
+  <version>1.6.1</version>
   <description>
     Efficient Task Space Inverse Dynamics (TSID) based on Pinocchio
   </description>