importing functions from image package to core

[Carnë Draug]

Hi

there's a bunch of simple functions on the image package that I think
could be moved to octave core (even though they used to belong to the
matlab image processing toolbox). These are the isbw, isgray, isind
and isrgb.

The reason I propose the move is that some functions in core already
perform this checks but have their own code. This functions are very
simple and would cause very little maintenance issues while reducing
repeated code.

If no one objects, I can prepare a changeset with them.

By the way, matlab removed this functions because of
http://blogs.mathworks.com/steve/2007/03/09/what-happened-to-isgray-isind-isbw-and-isrgb/
Not sure about the best approach there.

Carnë