Add antialiasing option for downsampling

[fepegar]

Related to #48.

Description

import torchio as tio

image = tio.datasets.Colin27().t1
image.plot()

factor = 8
downsampled_old = tio.Resample(factor, antialias=False)(image)
downsampled_old.plot()
downsampled_new = tio.Resample(factor, antialias=True)(image)
downsampled_new.plot()

Note e.g. the ventricles and basal ganglia in the sagittal view.

One more example with a test image. Note aliasing is smaller when enabling the new setting.

Checklist

• I have read the CONTRIBUTING docs and have a developer setup ready
• Changes are
  • Non-breaking (would not break existing functionality)
  • Breaking (would cause existing functionality to change)
• Tests added or modified to cover the changes
• In-line docstrings updated
• Documentation updated
• This pull request is ready to be reviewed

[fepegar]

@romainVala, could you please take a look?