Allow saving NULL values in saveout.net

[AdrienLeGuillou]

Just a thought, but since NULL could be a valid value we actually want to save, should we consider replacing things like out[[el.name]][[s]] <- dat[[el.name]] with out[[el.name]] <- c(out[[el.name]], list(dat[[el.name]]))? The latter appends dat[[el.name]] to out[[el.name]] even if dat[[el.name]] is NULL.

Originally posted by @chad-klumb in #790 (comment)

[chad-klumb]

There may also be an opportunity for some refactoring as we currently have duplication in saveout.net between the s == 1 and s > 1 cases.