Allow slicing with custom step size

[TheLartians]

Thanks for this great library!

This PR allows slicing datasets with a specified step size. This is extremely useful if we want to query a high resolution dataset using a lower sample rate than originally recorded, e.g. for performance / bandwidth reasons.

Example:

  const dset = write_file.create_dataset({
    name: "data", 
    data: [0,1,2,3,4,5,6,7,8,9], 
    shape: [10], 
    dtype: "<f4"
  });

 const slice = dset.slice([[null, null, 2]]); // -> [0,2,4,6,8]

[bmaranville]

Thanks for the very detailed PR - including tests! This will be a nice new feature.