Add the algorithm as an argument to `truncate!` for more customization

[mtfishman]

Add the algorithm as an argument to truncate! for more customization. Different algorithm backends might output the spectrum in different ways and therefore require different truncation implementations.

See also the discussion in #26.

[lkdvos]

I'm going to close this as not planned (for now), but I'm happy to reopen as soon as we find a case where this would solve an issue.

[mtfishman]

Agreed.