Add clean unstructured grid

[akaszynski]

Resolve #4345 by adding clean for UnstructuredGrid.

Also addresses the issues raised in #4357.

Demonstrate cleaning an UnstructuredGrid and show how it can be used to
average the point data across merged points.

>>> import pyvista as pv
>>> from pyvista import examples
>>> hexbeam = examples.load_hexbeam()
>>> hexbeam_shifted = hexbeam.translate([1, 0, 0])
>>> hexbeam.point_data['data'] = [0]*hexbeam.n_points
>>> hexbeam_shifted.point_data['data'] = [1]*hexbeam.n_points
>>> merged = hexbeam.merge(hexbeam_shifted, merge_points=False)
>>> cleaned = merged.clean(average_point_data=True)
>>> cleaned  # doctest:+SKIP
UnstructuredGrid (0x7f02930257e0)
  N Cells:    80
  N Points:   165
  X Bounds:   0.000e+00, 2.000e+00
  Y Bounds:   0.000e+00, 1.000e+00
  Z Bounds:   0.000e+00, 5.000e+00
  N Arrays:   5

Show how point averaging using the ``clean`` method with
``average_point_data=True`` results in averaged point data for merged
points.

>>> pl = pv.Plotter(shape=(1, 2))
>>> _ = pl.add_mesh(merged, scalars='data', show_scalar_bar=False)
>>> pl.subplot(0, 1)
>>> _ = pl.add_mesh(cleaned, scalars='data')
>>> pl.show()

[codecov]

Codecov Report

Merging #4367 (3c2a0f8) into main (caa1129) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #4367   +/-   ##
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  Coverage   95.81%   95.81%           
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  Files          97       97           
  Lines       20814    20828   +14     
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+ Hits        19942    19956   +14     
  Misses        872      872