Interpolation along particular axis

Currently, the interpolation function performs the interpolation along the 0th and the 1st axis. However, in my problem of interest, I need to interpolate along all the 4 axes. Could the feature of having interpolation along a particular axis be added on in `af.approx2`. Right now, I'm reordering the arrays such that the axes along which I need to interpolate are moved to the 0th and 1st axes. After which I need to reorder the resulting array to bring it back to the original convention.

This is what I'm currently doing right now:
```python
# vel_y_interpolant is stacked along axis 2
# vel_x_interpolant is stacked along axis 3
f_interp = af.approx2(af.reorder(f, 2, 3, 0, 1),\
af.reorder(vel_y_interpolant, 2, 3, 0, 1),\
af.reorder(vel_x_interpolant, 2, 3, 0, 1),\
af.INTERP.BICUBIC_SPLINE
)

f_interp = af.reorder(f_interp, 2, 3, 0, 1)
```

Start by reading the af.approx2 entry point and the issue's reorder-based example to understand the current axis assumptions. Check how interpolation arguments are exposed in the Python bindings. Done means af.approx2 can select the interpolation axes, including all four axes in the reported use case, without reordering the input and result arrays.

索引モデルが issue の本文から書いたものです。