* Refactor/test ToxFileProgress to ensure that when it's moved it
behaves well
* Notice problems with speed averaging. We were average speeds without
keeping track of the times they were over. Adding samples of different
lengths would result in incorrect speeds. Refactor whole class to correct
* Move ToxFileProgress to be a member of ToxFile
* Remove duplicated members between ToxFile and ToxFileProgress
* Move sample addition into CoreFile
