Appending data to existing dataset

[sebhinck]

I thought it might be useful for others as well to be able to add data to an already existing dataset.

These changes add this possibility to the Writer class. Instead of overwriting each file, one can specify append = True to open the existing file and append data.

[karimbahgat]

Thanks a lot for this, very interesting functionality that I think could be useful! I've been contemplating maybe an edit mode where one can also make use of the dbf deletion flag, in which case appending could be part of that. So will have to come back to this at a later point when I have time, to look over your code and do a little more thinking in terms of settling on an api.