Compute the equation of time

The equation of time is the difference of the real Sun time and the mean
Sun time. This difference is due to the elliptical shape of the Earth's
orbit, and the obliquity of the orbit.

`Sun` now has a `#equation_of_time` method that returns an integer: the
time difference in seconds.

```rb
time = Time.utc(2010, 7, 27, 12, 0, 0)
epoch = Astronoby::Epoch.from_time(time)
sun = Astronoby::Sun.new(epoch: epoch)

sun.equation_of_time
```

lib/astronoby/bodies/sun.rb

@@ -132,6 +132,20 @@ def orbital_eccentricity
       )
     end
 
+    def equation_of_time
+      date = Epoch.to_utc(@epoch).to_date
+      equatorial_hours = ecliptic_coordinates
+        .to_equatorial(epoch: @epoch)
+        .right_ascension
+        .hours
+      gst = GreenwichSiderealTime
+        .new(date: date, time: equatorial_hours)
+        .to_utc
+      noon = Time.utc(date.year, date.month, date.day, 12)
+
+      (noon - gst).to_i
+    end
+
     private
 
     def mean_anomaly

spec/astronoby/bodies/sun_spec.rb

@@ -572,4 +572,61 @@
       # Time from IMCCE: +266° 52′
     end
   end
+
+  describe "#equation_of_time" do
+    it "returns an Integer" do
+      time = Time.now
+      epoch = Astronoby::Epoch.from_time(time)
+      sun = described_class.new(epoch: epoch)
+
+      equation_of_time = sun.equation_of_time
+
+      expect(equation_of_time).to be_an Integer
+    end
+
+    # Source:
+    #  Title: Practical Astronomy with your Calculator or Spreadsheet
+    #  Authors: Peter Duffett-Smith and Jonathan Zwart
+    #  Edition: Cambridge University Press
+    #  Chapter: 51 - The equation of time
+    it "returns the right value of 2010-07-27" do
+      time = Time.utc(2010, 7, 27, 12, 0, 0)
+      epoch = Astronoby::Epoch.from_time(time)
+      sun = described_class.new(epoch: epoch)
+
+      equation_of_time = sun.equation_of_time
+
+      expect(equation_of_time).to eq(-392)
+    end
+
+    # Source:
+    #  Title: Celestial Calculations
+    #  Author: J. L. Lawrence
+    #  Edition: MIT Press
+    #  Chapter: 6 - The Sun
+    it "returns the right value of 2016-05-25" do
+      time = Time.utc(2016, 5, 5, 12, 0, 0)
+      epoch = Astronoby::Epoch.from_time(time)
+      sun = described_class.new(epoch: epoch)
+
+      equation_of_time = sun.equation_of_time
+
+      expect(equation_of_time).to eq(199)
+    end
+
+    # Source:
+    #  Title: Astronomical Algorithms
+    #  Author: Jean Meeus
+    #  Edition: 2nd edition
+    #  Chapter: 28 - Equation of Time
+    it "returns the right value of 1992-10-13" do
+      time = Time.utc(1992, 10, 13, 0, 0, 0)
+      epoch = Astronoby::Epoch.from_time(time)
+      sun = described_class.new(epoch: epoch)
+
+      equation_of_time = sun.equation_of_time
+
+      expect(equation_of_time).to eq(937)
+    end
+  end
 end