Add one preliminary step for NEB

@mikibonacci and Ifeanyi and working on supporting NEB calculations as well. They as quite similar, except that one would need to pick (at least) 2 structures instead of 1, and possibly 1/2 more parameters (number of images) , and which atoms to fix (but this is more general and one should support it already now, also just for relax)

Some things could be supported just with plug-ins (e.g showing the final energy profile, the final structures that the neb finds, etc) and some slightly more complex workflow behind the scenes, but I think it's a bit strange to pick one structure normally, and the second one in a different tab as a plug-in.

Would it make sense to have a very first step that asks if we want to do NEB or single-image calculations? In the second case, we continue with the current steps. In the first case (NEB), the following step (structure selection) will habe 2 selection widgets, and possibly the few remaining parameters such as number of NEB images. And validation that the number of atoms and atomic species are the same etc.

The disadvantage I see is that for most people there is one more step that 99% of the cases is always the same, but one could pick as a default the non-NEB option, so it's just a click.

The advantage is avoiding that, to support NEB, we start copy pasting the whole QE app just to do a NEB app with minor modifications.

Ideas welcome @cpignedoli @superstar54 @edan-bainglass @AndresOrtegaGuerrero