series_test():
  Demonstrate the use of Series in pandas.

  Create a series:
0    6300
1    6650
2    6275
3    6400
dtype: int64

  Print with a loop "for i in range ( 0, 4 ):"
6300
6650
6275
6400

  Print with a generator "for length in river_lengths":
6300
6650
6275
6400

  Print river_lengths.index:
RangeIndex(start=0, stop=4, step=1)

  Specify river names as the index, and print river_lengths:
Yangtze        6300
Nile           6650
Mississippi    6275
Amazon         6400
dtype: int64

  Use a river name to index a river length:
  river_lengths['Nile'] =  6650

  Specify index, name and dtype.

  Now print river_lengths
Yangtze        6300.0
Nile           6650.0
Mississippi    6275.0
Amazon         6400.0
Name: Length (km), dtype: float64

  Now print river_lengths["Nile"]
6650.0

  Now print river_lengths["Yangtze":"Mississippi"]
Yangtze        6300.0
Nile           6650.0
Mississippi    6275.0
Name: Length (km), dtype: float64

  Filter the data.
  print ( river_lengths[river_lengths < 6500] ):
Yangtze        6300.0
Mississippi    6275.0
Amazon         6400.0
Name: Length (km), dtype: float64

  Make a new Series, "river_lengths_miles"
  river_lengths_miles = river_lengths * 0.621371
  river_lengths_miles.name = "Length (miles)"
Yangtze        3914.637300
Nile           4132.117150
Mississippi    3899.103025
Amazon         3976.774400
Name: Length (miles), dtype: float64

  Series for mass of astronomical bodies:
Ganymede    1.482000e+23
Callisto    1.076323e+00
Io          8.932000e+22
Europa      4.800000e+22
Moon        7.342000e+22
Earth       5.972000e+24
Name: mass (kg), dtype: float64

  Series for radius of astronomical bodies:
Ganymede    2634000.0
Io          1822000.0
Moon        1737000.0
Earth       6371000.0
Name: radius (m), dtype: float64

  Create new series "gravity = G * mass / radius**2"

Body
Callisto         NaN
Earth       9.819973
Europa           NaN
Ganymede    1.425681
Io          1.795799
Moon        1.624129
Name: surface gravity m/s**2, dtype: float64

  gravity.isnull() checks for NaN values.
Body
Callisto     True
Earth       False
Europa       True
Ganymede    False
Io          False
Moon        False
Name: surface gravity m/s**2, dtype: bool

  gravity = gravity.dropna() drops NaN entries.
Body
Earth       9.819973
Ganymede    1.425681
Io          1.795799
Moon        1.624129
Name: surface gravity m/s**2, dtype: float64

  gravity_array = gravity.values creates numpy array.
[9.81997343 1.42568108 1.79579887 1.6241288 ]