Draft: Adding an option to export the data for each channel to ascii

I think the function added in this commit needs to be called from the NDScope class, but I'm not sure how to access it from there without making large changes. However, this seems like the most appropriate place to put this function. If it took the data from each curve instead, then it would export any scale and offset transformations that have been applied. I'm creating this draft merge request to ask for advice on how to move forward.

Relates to issue #272.