Input validation overhaul

[roaldarbol]

Currently it's become a bit sporadic, with some ensure_ and validate_ functions, and then also lots of loose input validations happening, especially in newer functions. I should return to such functions, streamline them and use them across the package.

Before going too far, it's worth checking whether the validate package, might be the right tool for this (might also work well for the general data checks!).

Maybe ensure_ and/or validate_ functions should have a parameter to either return TRUE/FALSE or return error/pass, to avoid the need for extra functions when a boolean output is needed.