Reduce code duplication

Author: VeckoTheGecko

parcels/_interpolation.py

@@ -296,28 +296,17 @@ def _linear_3d(ctx: InterpolationContext3D) -> float:
 @register_3d_interpolator("bgrid_velocity")
 def _linear_3d_bgrid_velocity(ctx: InterpolationContext3D) -> float:
     if ctx.gridindexingtype == "mom5":
-        zeta = 1.0
+        ctx.zeta = 1.0
     else:
-        zeta = 0.0
-    eta = ctx.eta
-    xsi = ctx.xsi
-    zdim = ctx.data.shape[1]
-    data_3d = ctx.data[ctx.ti, :, :, :]
-    f0, f1 = get_3d_f0_f1(eta=eta, xsi=xsi, data=data_3d, zi=ctx.zi, yi=ctx.yi, xi=ctx.xi)
-
-    return z_layer_interp(zeta=zeta, f0=f0, f1=f1, zi=ctx.zi, zdim=zdim, gridindexingtype=ctx.gridindexingtype)
+        ctx.zeta = 0.0
+    return _linear_3d(ctx)
 
 
 @register_3d_interpolator("bgrid_w_velocity")
 def _linear_3d_bgrid_w_velocity(ctx: InterpolationContext3D) -> float:
-    zeta = ctx.zeta
-    eta = 1.0
-    xsi = 1.0
-    zdim = ctx.data.shape[1]
-    data_3d = ctx.data[ctx.ti, :, :, :]
-    f0, f1 = get_3d_f0_f1(eta=eta, xsi=xsi, data=data_3d, zi=ctx.zi, yi=ctx.yi, xi=ctx.xi)
-
-    return z_layer_interp(zeta=zeta, f0=f0, f1=f1, zi=ctx.zi, zdim=zdim, gridindexingtype=ctx.gridindexingtype)
+    ctx.eta = 1.0
+    ctx.xsi = 1.0
+    return _linear_3d(ctx)
 
 
 @register_3d_interpolator("bgrid_tracer")