Improve ObsPy support so that we can include SAC files

Created by: emthompson-usgs

Our initial goal of the "fdsn" format was for it to handle all ObsPy formats. That said, we discovered that many of the formats do not contain sufficient metadata and so we ended up only really supporting miniseed + StationXML. It turns out that SAC headers do not have an entry to indicate units (only unit type in the "IDEP" field). So to overcome this issue, I added a field in the config for the user to specify the conversion factor from the units in the SAC file to cm/s/s (or cm/s). I have not yet added support for SAC files in raw counts where the instrument response is given in a PZ file, but that should be relatively easy to do eventually (and in that case, the config conversion unit is probably not necessary).

Also, I modified the Turkey fetcher so that it doesn't raise an exception when no events are found. It appears that they have modified their website since we wrote the fetcher and so it probably needs to be revisited soon.