A Variant-Based LoadChunk

[ax3l]

It would be great if we can design load routines that are variant-based.

That way, the user-side logic is more concise than knowing the exact type upfront.

Similar for attribute reads (where we already support some flexible casts).

[franzpoeschel]

The logic of our switchType thingy is very similar to std::visit and can be used today already by including DatatypeHelpers.hpp (public header on purpose). Maybe a wrapper around this would be a solution?
Otherwise, to use std::visit (which would be nice as it's well-documented STL code), we would have to use an actual std::variant somewhere, but it's probably doable too.

[ax3l]

Things we could add:

• public std::variant for our users for our defined types
• return overloads that return a pre-allocated variant
• helpers so that users can pass us a pre-allocated float (double, etc.) pointer and we write into it, converting if needed