Massive change to DiscreteSeries. Wrote a large number of GSL interpolation function wrappers, added exceptions, wrote a few tests (they're terrible at the moment but will be improved shortly), added a large chunk of the functionality which std::valarray has (the slices may or may not be implemented in a reasonable amount of time, not sure yet), added boost::zip_iterator support so that (x,y) pair-wise iteration over DiscreteSeries is possible, and reduced the amount of boilerplate code. The next update will be focused on removing dead code, improving comments for doxygen, and perhaps more tests.