Add ".xyz" extension to PointSet.save

[tkoyama010]

Overview

PointSet.save cannot be written because _WRITER does not exist, and an error always occurs. Therefore, in this PR, I have clarified which extension to specify by defining _WRITER. IMO, the only Writer object that can be used with PointSet is vtkSimplePointsWriter, which creates xyz file. (If there are other Writers that can be added, please point them out.)

resolves #2556

Details

import numpy as np
import pyvista

rng = np.random.default_rng()
points = rng.random((10, 3))
pset = pyvista.PointSet(points)
pset.save("pset.xyz")

[codecov]

Codecov Report

Merging #2571 (6da1a7f) into main (c49a6ab) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2571   +/-   ##
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  Coverage   93.69%   93.69%           
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  Files          75       75           
  Lines       16159    16160    +1     
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+ Hits        15140    15141    +1     
  Misses       1019     1019           

[MatthewFlamm]

This is a good idea. What about this writer? vtkSimplePointsWriter. Edit: I somehow completely misread this.

It would also be nice to have a reader for xyz files and we could test the roundtrip. vtkSimplePointsReader

[MatthewFlamm]

One problem I see with this format is that I don't think that any point_data will get saved. Maybe the best we can do is writing it to vtp or the xml version?

[tkoyama010]

Thanks for the comment. As you say we can casting PointSet to PolyData.
I will improve this PR.
Edit: I noticed that we can output to vtk by using the following expression. I considered doing this internally in PoinSet class but concluded that "as is" is the best way to avoid confusing the user.

pset.cast_to_polydata().save("pset.vtk")

[tkoyama010]

Anyway, it would be useful to add xyz file output.

[akaszynski]

This is good to merge as-is. We can add a specialized reader as a next step.