More SPECT features

[saarlemo]

Orthogonal distance ray tracing projector for SPECT

• Assumes circular holes for now
• A voxel is weighed by the parallel and orthogonal distance from the detector element center normal
• The weight is calculated from a Gaussian evaluated at the orthogonal distance, with variance dependent on the parallel distance
• CPU, OpenCL and CUDA

SPECT reconstructions with matrix operations ($Ax$ forward projection and $A^Ty$ backprojection)

• MATLAB and Python
• The examples include a single pixel backprojection and single voxel forward projection

Rotation-based projector FOV resolution is no longer constrained by sinogram resolution

• Sinograms are resized and resampled to match FOV XZ-plane
• Cubical voxels still necessary

The function volume3Dviewer now has support for PET/CT and SPECT/CT multimodal overlays (MATLAB only as Octave has no support for the alpha function).

Miscellaneous improvements

• Fixed typos
• Macros for OpenCL / CUDA status checks
• Reformatted the wall of bullet points in readme