Add cbind and rbind methods

[jranke]

Fixes #311

[Enchufa2]

Thanks. This PR currently breaks quantities. I'd like to avoid that unless strictly necessary. Did you look into it by any chance?

[jranke]

Thanks. This PR currently breaks quantities. I'd like to avoid that unless strictly necessary. Did you look into it by any chance?

I had a quick look, but it gets a bit complicated. The proposed method cbind.units is called in errors:::propagate in the case of quantities and there it fails, because not all arguments have units. If the check for this is removed, it still fails, presumably because it does not handle the error attributes. I do not understand the interaction of the packages and classes well enough to solve this.

[Enchufa2]

No problem. Let's put this on hold then until I find the time to have a look.