Validate/sanitize inputs more generally

[thomased]

We've got some specific checks around, but might be nice to have some general checks & a generic error ('Input not recognised') that just catches all totally-wrong inputs. All the functions expect pretty specific stuff, so an internal checker function or two, that's then called near the beginning of main functions, would probably do it.

[Bisaloo]

I totally agree! Many functions use the same first lines of code and it would be great to refactor it to reduce code redundancy. I think the main challenge here is to stay flexible enough when checking the input. For example, I tend to work a lot with dataframes instead of dedicated S3 classes. There's nothing more annoying than functions that fail without explanations because you didn't provide the exact input they expect (matrices vs df, named rows, etc.).