Fixed C++ extensions compiler error

[lgleznah]

Attempting to run the experiment scripts with the instructions given in the Setup section of Standalone/README.md results in multiple issues, of which, the most notable is a series of compiler errors of the C++ extensions modules, all of which were of the form cannot convert ‘PyObject*’ {aka ‘_object*’} to ‘const PyArrayObject*’ {aka ‘const tagPyArrayObject_fields*’}

I have fixed this by adding casts to PyArrayObject* in the places where errors were reported. This, together with some more fixes I have described in the aforementioned README.md, has allowed me to properly run the training script.

[Paco-Ruiz-Pino]

do you fix this error:
File "ml-kpconvx/Standalone/KPConvX/utils/cpp_funcs.py", line 87, in batch_grid_partition
s_points, s_len, pools, ups = cpp_subsampling.batch_grid_partitionning(points,
AttributeError: module 'cpp_wrappers.cpp_subsampling' has no attribute 'batch_grid_partitionning'?

[lgleznah]

@Paco-Ruiz-Pino Hmm, that's odd, I don't remember getting that error, at least when running the S3DIS demo.

Under which conditions did you get that error? How did you compile the C++ extensions?