Fix Quad Transport Map w/ Slicing and Zero Fields (#1038)

The linear transport map of the quad, used in envelope
tracking mode, did not scale its lengths properly when
set to zero field strength and sliced for space charge
effects. This fixes it.

Author: Kyrre Ness Sjobak <[email protected]>

Co-authored-by: Kyrre Ness Sjobak <[email protected]>

Author: ax3l

src/elements/Quad.H

@@ -4,7 +4,7 @@
  *
  * This file is part of ImpactX.
  *
- * Authors: Chad Mitchell, Axel Huebl
+ * Authors: Chad Mitchell, Axel Huebl, Kyrre Ness Sjobak
  * License: BSD-3-Clause-LBNL
  */
 #ifndef IMPACTX_QUAD_H
@@ -274,9 +274,9 @@ namespace impactx::elements
                 R(4,4) = std::cos(omega*slice_ds);
                 R(5,6) = slice_ds/betgam2;
             } else {
-                R(1,2) = m_slice_ds;
-                R(3,4) = m_slice_ds;
-                R(5,6) = m_slice_ds / betgam2;
+                R(1,2) = slice_ds;
+                R(3,4) = slice_ds;
+                R(5,6) = slice_ds / betgam2;
             }
 
             return R;