ImgPlus: undeprecate getImg method

Christian Dietz rightly points out that there are legitimate use cases
for obtaining the backing Img object. However, it is good to remind
people that in the usual cases, you do not need getImg() but rather can
use the ImgPlus directly.
Bump to next development cycle

Signed-off-by: Barry DeZonia <bdezonia@wisc.edu>
Merge branch 'origin/master'