Replaced set_variable by set_string_variable

Author: PhilippHoellmer

test_driver/run_length_control.py

@@ -617,7 +617,7 @@ def get_trajectory(step: int, args: dict) -> np.ndarray:
     cmd = "variable run_var string ''"
     lmp.command(cmd)
 
-    lmp.set_variable('run_var', msg)
+    lmp.set_string_variable('run_var', msg)
 
     cmd = 'print "${run_var}"'
     lmp.command(cmd)