Add new split_bodies filter

[banesullivan]

Adds the ability to split all the different connected volumes/bodies in a dataset. This example is in the docs:

import numpy as np
import vtki
from vtki import examples
vtki.set_plot_theme('document')

# Load a simple example mesh
dataset = examples.load_uniform()
dataset.set_active_scalar('Spatial Cell Data')
threshed = dataset.threshold_percent([0.15, 0.50], invert=True)

bodies = threshed.split_bodies()

for i, body in enumerate(bodies):
    print('Body {} volume: {:.3f}'.format(i, body.volume))

bodies.plot(show_bounds=True, multi_colors=True)

Body 0 volume: 518.000
Body 1 volume: 35.000

[codecov-io]

Codecov Report

Merging #104 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
+ Coverage   89.98%   90.05%   +0.06%     
==========================================
  Files          17       17              
  Lines        4294     4312      +18     
==========================================
+ Hits         3864     3883      +19     
+ Misses        430      429       -1

Impacted Files	Coverage Δ
vtki/filters.py	99.66% <100%> (+0.02%)	⬆️
vtki/utilities.py	94.07% <0%> (+0.65%)	⬆️

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