Check that files are not half downloaded

[garezana]

No description provided.

[colinsauze]

If the offlinedatasci process is stopped halfway through downloading a file, running it a second time doesn't cause the rest of the file to be downloaded. There needs to be a check of each file's downloaded size and the size of source file to see if it the download should be redone (or better still continued).

[abhidg]

wget calls are using -c so continuation should work, will check

[ethanwhite]

@abhidg - yes, that's true for lessons, but not for other things. @garezana & I have discussed plans for not writing incomplete downloads and retrying (see also #46) that should work for everything except miniCRAN.