TransformixFilter should _not_ automatically do this->SetOutputDirectory(".")

[N-Dekker]

Users may leave the output directory path empty, which is usually an indication that no output is written, However when ComputeSpatialJacobian, ComputeDeterminantOfSpatialJacobian, ComputeDeformationField, or FixedPointSetFileName are set, TransformixFilter will still try to write to the current directory.

TransformixFilter::GenerateData() does:

elastix/Core/Main/itkTransformixFilter.hxx

Lines 112 to 117 in c47889a

	if ((m_ComputeSpatialJacobian || m_ComputeDeterminantOfSpatialJacobian || m_ComputeDeformationField ||
	!m_FixedPointSetFileName.empty() || m_LogToFile) &&
	m_OutputDirectory.empty())
	{
	this->SetOutputDirectory(".");
	}

It may be a deliberate choice to not write to an output directory. So TransformixFilter should not do this->SetOutputDirectory("."). Instead I think it would be better if it would just produce a warning message, like:

Output directory not specified. Unable to write SpatialJacobian, DeterminantOfSpatialJacobian, DeformationField, or FixedPointSet to file.

What do you think?