Make small changes

src/plot_analytic_solution.py

@@ -6,7 +6,7 @@ def main():
     x = []
     v = []
 
-    with open('analytical_solution.txt') as f:
+    with open('output/analytical_solution.txt') as f:
         for line in f:
             a, b = line.strip().split()
             x.append(float(a))