RNifti

• Assigning pixel dimensions with pixdim<- now produces an error if the replacement value's length doesn't match the image's dimensionality, rather than silently applying it incorrectly.
• Setting the world-coordinate transformation of a niftiHeader object with qform<- or sform<- no longer has the side effect of overwriting its pixel dimensions, except for the sign of the first element (the "qfac").
• writeNifti()'s version argument is now validated, and may be given as a string ("1" or "2") as well as numerically.
• Assigning to imageAttributes() now produces a warning, rather than silently doing nothing, if the assignment would overwrite a core NIfTI attribute. It also now works correctly when the target object has no pre-existing attributes.
• The mean() method for internal images now uses the same C++ back-end as the Summary group generic, thereby avoiding first converting the image to a full array.

API changes

• A bug in the acquire()/release() reference-counting cycle in the NiftiImage C++ class has been fixed. Previously, changing the pointer wrapped by a NiftiImage object by copy-assignment could decrement the wrong reference counter, risking a premature deallocation. acquire() now takes an optional existing reference counter to take over, rather than relying on it having already been set as a side effect by the caller.
• Converting a NIfTI-2 image containing more voxels than a NIfTI-1 image can represent now fails with an informative error, rather than silently overflowing.

Other changes

• Reading corrupt or truncated image data now produces clearer, more informative errors.
• A memory leak affecting the internal per-datatype handler on copy assignment of a NiftiImage, and undefined behaviour arising from use of an iterator after erasing the element it points to, have both been fixed.
• C++ exceptions are now consistently raised using Rcpp::stop() rather than Rf_error(), ensuring correct stack unwinding.