Download, clean, reformat and create new variables from the
    USA National Climatic Data Center (NCDC) Global Surface Summary of the Day
    (GSOD) weather stations data,
    <https://data.noaa.gov/dataset/global-surface-summary-of-the-day-gsod>.
    The function, get_GSOD(), retrieves data from the GSOD ftp site and
    reformats it from United States Customary System (USCS) units to
    International System of Units (SI). Stations are individually checked for
    number of missing days, as defined by the user, to assure data quality.
    Stations with too many missing observations are omitted from final file.
    Stations with missing latitude or longitude or values for both of 0 are
    omitted. Also omitted are stations with a latitude of < -90 or > 90 or
    longitude of < - 180 or > 180. Output is returned as a comma-separated
    values (CSV) file written to disk in a location selected by the user, which
    summarises each year by station and includes new variables: actual vapor
    pressure, saturation vapor pressure and relative humidity, calculated
    from the original GSOD data. The resulting files can be as large as 500mb
    depending on the user's stringency for missing data and geographic area of
    interest. Be sure to have sufficient RAM and disk space as well as a
    reasonably fast internet connection to use this package to perform this
    operation. However, for much smaller and more manageable data sets, an
    individual country of interest may be selected as well as only stations
    falling between -60/60 degrees latitude for agroclimatology work or
    individual stations if the station ID is known. The resulting files include
    station data (e.g., station name, country, latitude, longitude, elevation)
    for use in a geographic information system (GIS). The function was largely
    based on T. Hengl's 'getGSOD.R' script, available from
    <http://spatial-analyst.net/book/system/files/getGSOD.R> with enhancements
    to be cross-platform, faster and more flexible. For information on the
    original data from NCDC, please see the GSOD readme.txt file available from,
    <http://www1.ncdc.noaa.gov/pub/data/gsod/readme.txt>.