Fix compute_and_save_png_slices binding default parameter value to match parameter type

[Nolan1324]

In the Python bindings file python_api.cu, the binding for compute_and_save_png_slices has a mistake. The binding contains py::arg("resolution") = ivec3(256) which sets the default value of this argument to a value of type ivec3. However, compute_and_save_png_slices has the following function declaration in testbed.h

ivec3 compute_and_save_png_slices(const fs::path& filename, int res, BoundingBox aabb = {}, float thresh = 2.5f, float density_range = 4.f, bool flip_y_and_z_axes = false);

This reveals that the correct type of res is in fact int for this function, rather than ivec3.

This can cause some confusion and errors. For example, if arguments of incorrect type are passed to the binding, the following error is thrown:

TypeError: compute_and_save_png_slices(): incompatible function arguments. The following argument types are supported:
    1. (self: pyngp.Testbed, filename: pyngp.path, resolution: int = array([256, 256, 256]), aabb: pyngp.BoundingBox = <pyngp.BoundingBox object at 0x0000029235215F70>, thresh: float = 3.4028234663852886e+38, density_range: float = 4.0, flip_y_and_z_axes: bool = False) -> vec

This could make it seem that the type of resolution is an array, when in fact it is a scalar.

To resolve this, I request changing the default value in the binding definition from ivec3(256) to 256.

[Tom94]

Thanks for the fix -- and apologies for the delay.