Fix data type for GetTimeSpacing method.
Fix padding issue with sizeof(nifti_2_header).

Because nifti_2_header contains data types that require 8-byte alignment,
the sizeof() operator pads its size up to a multiple of eight bytes.  That
means that sizeof() returns a value that is 4 bytes larger than the nominal
size of 540 bytes.
Perform flush-to-zero on matrix computed from quaternion.

The quaternion-to-matrix computation has a certain amount of numerical
error associated with it.  Since the desired matrix is usually a permutation
matrix, it is a good idea to convert any miniscule matrix elements to zero
and re-normalize, which effectively eliminates roundoff error in the majority
of useful cases.