Change TWeights in MultiLabelSTAPLE to double

[thomas-albrecht]

The WeightsType TWeights of the itk::MultiLabelSTAPLEImageFilter is set to the default float.
There is one big problem with this.

In itkMultiLabelSTAPLEImageFilter.hxx
the frequency of the labels is computed like this:

      for (in.GoToBegin(); !in.IsAtEnd(); ++in)
      {
        ++(this->m_PriorProbabilities[in.Get()]);
      }

where this->m_PriorProbabilities is of course an Array<WeightsType>.
Ultimately, the code is doing this:

float frequency = 0;
for (int i = 0; i < num_voxels; i++) {
  frequency = frequency + 1
}

At some point, precisely at $frequency = 2^{24} = 16,777,216$, the float representation of frequency and frequency + 1 is the same and the counter stops increasing. This number easily reached in even a modest 3D images, and the whole m_PriorProbabilities will be completely wrong.

Short of changing the ITK code, a simple fix is to use double as the weights type, which raises this problematic threshold to $2^{53} = 9,007,199,254,740,992$ (over 9 quadrillion).