coregisterImages tool manual page uses contentious language

[MatthiasSiewert]

The terms 'master ' and 'slave' should be replaced by neutral terms to reflect current societal development.

[bleutner]

While the cause is just, I don't think this is going to make any difference whatsoever. In my opinion, such cosmetic activism is a distraction from the actual issues which would much rather need to be addressed. Nevertheless, I'm aware that some people feel and think otherwise, hence I followed your suggestion and renamed the arguments of master and slave to ref and img, respectively.