Make align and friends multivariate

Now, that we provide multivariate functions, that work on sets of variables, I start to miss an easy-to-use possibility to 'harmonize' a bunch of variables, i.e. bring several variables to the same index and optionally interpolate or fill newly added index values. We already provide many functions to resample single variables, but I couldn't find one to do so over several variables.

Maybe we don't even need to provide new functions, but instead reuse the ideas discussed in #295 (closed) and make the existing resampling functions:

1. multivariate
2. and optionally work row wise

Even if we never end up making everything work row wise like proposed in #295 (closed) maybe the resampling methods could anyhow benefit from this and therefore also be sort of an proof of concepts.