fix Machnumber field spelling in amrex frontend for Castro

also add abar and Ye as defined fields so they print well

yt/frontends/amrex/fields.py

@@ -328,7 +328,9 @@ class CastroFieldInfo(FieldInfoContainer):
             ("erg/cm**3", ["kinetic_energy_density"], r"\frac{1}{2}\rho|\mathbf{U}|^2"),
         ),
         ("soundspeed", ("cm/s", ["sound_speed"], "Sound Speed")),
-        ("Machnumber", ("", ["mach_number"], "Mach Number")),
+        ("MachNumber", ("", ["mach_number"], "Mach Number")),
+        ("abar", ("", [], r"$\bar{A}$")),
+        ("Ye", ("", [], r"$Y_e$")),
         ("entropy", ("erg/(g*K)", ["entropy"], r"s")),
         ("magvort", ("1/s", ["vorticity_magnitude"], r"|\nabla \times \mathbf{U}|")),
         ("divu", ("1/s", ["velocity_divergence"], r"\nabla \cdot \mathbf{U}")),