Make labels with math text raw strings since they behave better (#5740)

This fixes one analysis file. There are probably others with the same
issue. Note that using raw strings is only needed when the strings
include back slashes, but for consistency all labels with math text are
made raw.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Examples/Tests/ohm_solver_ion_beam_instability/analysis.py

@@ -70,10 +70,10 @@
 # Colorbar
 fig.subplots_adjust(right=0.825)
 cbar_ax = fig.add_axes([0.85, 0.2, 0.03, 0.6])
-fig.colorbar(im, cax=cbar_ax, orientation="vertical", label="$B_y/B_0$")
+fig.colorbar(im, cax=cbar_ax, orientation="vertical", label=r"$B_y/B_0$")
 
-ax1.set_xlabel("$x/l_i$")
-ax1.set_ylabel("$t \Omega_i$ (rad)")
+ax1.set_xlabel(r"$x/l_i$")
+ax1.set_ylabel(r"$t \Omega_i$ (rad)")
 
 ax1.set_title(f"Ion beam R instability - {resonant_str} case")
 plt.savefig(
@@ -88,13 +88,13 @@
 
     t_grid = np.arange(num_steps) * dt * sim.w_ci
     plt.plot(
-        t_grid, np.abs(field_kt[:, 4] / sim.B0), "r", label=f"m = 4, $kl_i={k[4]:.2f}$"
+        t_grid, np.abs(field_kt[:, 4] / sim.B0), "r", label=rf"m = 4, $kl_i={k[4]:.2f}$"
     )
     plt.plot(
-        t_grid, np.abs(field_kt[:, 5] / sim.B0), "b", label=f"m = 5, $kl_i={k[5]:.2f}$"
+        t_grid, np.abs(field_kt[:, 5] / sim.B0), "b", label=rf"m = 5, $kl_i={k[5]:.2f}$"
     )
     plt.plot(
-        t_grid, np.abs(field_kt[:, 6] / sim.B0), "k", label=f"m = 6, $kl_i={k[6]:.2f}$"
+        t_grid, np.abs(field_kt[:, 6] / sim.B0), "k", label=rf"m = 6, $kl_i={k[6]:.2f}$"
     )
 
     # The theoretical growth rates for the 4th, 5th and 6th Fourier modes of
@@ -119,8 +119,8 @@
     plt.grid()
     plt.legend()
     plt.yscale("log")
-    plt.ylabel("$|B_y/B_0|$")
-    plt.xlabel("$t\Omega_i$ (rad)")
+    plt.ylabel(r"$|B_y/B_0|$")
+    plt.xlabel(r"$t\Omega_i$ (rad)")
     plt.tight_layout()
     plt.savefig(
         f"diags/ion_beam_R_instability_{resonant_str}_eta_{sim.eta}_substeps_{sim.substeps}_low_modes.png"
@@ -172,11 +172,11 @@
     fig.subplots_adjust(bottom=0.15, right=0.815)
     cbar_ax = fig.add_axes([0.83, 0.2, 0.03, 0.6])
     fig.colorbar(
-        im[3], cax=cbar_ax, orientation="vertical", format="%.0e", label="$f(z, v_y)$"
+        im[3], cax=cbar_ax, orientation="vertical", format="%.0e", label=r"$f(z, v_y)$"
     )
 
-    ax1.set_xlabel("$x/l_i$")
-    ax1.set_ylabel("$v_{y}/v_A$")
+    ax1.set_xlabel(r"$x/l_i$")
+    ax1.set_ylabel(r"$v_{y}/v_A$")
 
     ax1.set_title(f"Ion beam R instability - {resonant_str} case")
     plt.savefig(
@@ -207,11 +207,11 @@
     fig.subplots_adjust(bottom=0.15, right=0.815)
     cbar_ax = fig.add_axes([0.83, 0.2, 0.03, 0.6])
     fig.colorbar(
-        im[3], cax=cbar_ax, orientation="vertical", format="%.0e", label="$f(z, v_y)$"
+        im[3], cax=cbar_ax, orientation="vertical", format="%.0e", label=r"$f(z, v_y)$"
     )
 
-    ax1.set_xlabel("$x/l_i$")
-    ax1.set_ylabel("$v_{y}/v_A$")
+    ax1.set_xlabel(r"$x/l_i$")
+    ax1.set_ylabel(r"$v_{y}/v_A$")
 
     ax1.set_title(f"Ion beam R instability - {resonant_str} case")
     plt.savefig(