bug in remove_artifacts.py, can't handle sparsity

[maxjuv]

Hello,
I was using the remove_artifacts function in "median" mode, and I was playing with the parameter "sparsity". I got an error because the sparsity was not applied on the artifact template. Indeed line 185-187 , the sparsity is not taken into account. I fixed it locally with the following code. If the solution is fine for you, I can create a pull request.

artifacts = {}
for i, label in enumerate(sorting.unit_ids):
    if sparsity is not None :
        artifacts[label] = templates[i, :, :][:,sparsity[label]]
    else :
        artifacts[label] = templates[i, :, :]

Detailed error :
I made sparsity masks conserving only 1 channel. My recording is composed of 192 channels. I got the following error :

 np.dot(trace_slice_values.flatten(), artifact_slice_values.flatten()) / norm
  File "<__array_function__ internals>", line 200, in dot
ValueError: shapes (18,) and (3456,) not aligned: 18 (dim 0) != 3456 (dim 0)

It happened that 3456 is exactly 192*18. So I tracked the shape of "artifact_slice_values", and I observed that sparsity mask was not applied on the template of the artifacts.
Cheers