do not rely on implicit string concatenation

Author: amorison

src/stagpy/plates.py

@@ -417,7 +417,7 @@ def cmd(conf: Config) -> None:
     with open(f"{oname}.dat", "w") as fid:
         fid.write(
             "#  istep     time   time_My   phi_trench  vel_trench  "
-            "distance     phi_cont  age_trench_My\n"
+            + "distance     phi_cont  age_trench_My\n"
         )
 
         for step in view.filter(fields=["T"]):

src/stagpy/processing.py

@@ -263,7 +263,7 @@ def stream_function(step: Step) -> Field:
         shape = (v_x.shape[0], 1, v_x.shape[1], 1)
     else:
         raise NotAvailableError(
-            "Stream function only implemented in " "2D cartesian and spherical annulus"
+            "Stream function only implemented in 2D cartesian and spherical annulus"
         )
     psi = np.zeros_like(v_x)
     if step.geom.spherical:  # YZ annulus