data.table loaded from disk using load() causes unexpected error message when immediately used in intervalaverage function

[myoung3]

This was fixed in is.overlapping by replacing the walrus operator (:=) with a set function in savestate (the set function returns an intelligible error message whereas the walrus operator when applied to a data.table loaded from disk may silently fail to add the column).

there are more walrus operators in the intervalaverage function that either need to be replaced with calls to the set function or else you need to ensure that memory is allocated for the data.table before proceeding with the intervalaverage function.

It would be great to write a unit test for this but it's slightly involved since you need to write a file to disk and include it with the package.