DOC: tiny comment edit
API: set offset to zero when creating images

For Analyze types, treat the file offset as consumable. So, set to zero
when creating an image object, and set to the necessary (mimumum) offset
when writing the header.

If the user sets the offset directly to a value other than zero, use
that value or raise and error if it is too small.
RF: make default offset 0 for nifti headers

Set default offset to 0 (signalling - 'do what you need to do') rather than
setting offset to allow for length of header in single image.
BF: make SPM image offset checks depend on scipy

We were getting errors with scipy not installed; guard offset checks
with checks for scipy.
BF: fix missed extension status check on Python 3

Python 3 was doing the check wrong for extension status == 0; integer
slicing into a byte array results in an integer, so comparison of this
integer against a byte string was always false.  This is OK as long as
the offset is default, but gives an error later if the offset is higher
than the default minimum.