updated the Astronomical unit system

- updated the default units (as per https://en.wikipedia.org/wiki/Astronomical_system_of_units)
- fixed the docstring

Author: lipchev

Common/UnitDefinitions/Duration.json

@@ -74,7 +74,7 @@
       },
       "FromUnitToBaseFunc": "x*24*3600",
       "FromBaseToUnitFunc": "x/(24*3600)",
-      "UnitSystems": [ "SI", "CGS", "BI", "EE", "USC", "FPS" ],
+      "UnitSystems": [ "SI", "CGS", "BI", "EE", "USC", "FPS", "Astronomical" ],
       "Localization": [
         {
           "Culture": "en-US",
@@ -175,7 +175,7 @@
     },
     {
       "UnitSystem": "Astronomical",
-      "BaseUnit": "Second"
+      "BaseUnit": "Day"
     }
   ]
 }

Common/UnitDefinitions/Length.json

@@ -387,7 +387,7 @@
     },
     {
       "UnitSystem": "Astronomical",
-      "BaseUnit": "LightYear"
+      "BaseUnit": "AstronomicalUnit"
     }
   ]
 }

Common/UnitDefinitions/Mass.json

@@ -286,7 +286,7 @@
     },
     {
       "UnitSystem": "Astronomical",
-      "BaseUnit": "Kilogram"
+      "BaseUnit": "SolarMass"
     }
   ]
 }

UnitsNet/CustomCode/UnitSystems/Astronomical.cs

@@ -3,18 +3,14 @@
 namespace UnitsNet.UnitSystems
 {
     /// <summary>
-    ///     The centimetre–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the
-    ///     centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS
-    ///     mechanical units are unambiguously derived from these three base units, but there are several different ways of
-    ///     extending the CGS system to cover electromagnetism.[1][2][3]
-    ///     The CGS system has been largely supplanted by the MKS system based on the metre, kilogram, and second, which was in
-    ///     turn extended and replaced by the International System of Units(SI). In many fields of science and engineering, SI
-    ///     is the only system of units in use but there remain certain subfields where CGS is prevalent.
+    ///     The astronomical system of units, formerly called the IAU System of Astronomical Constants, is a system of
+    ///     measurement developed for use in astronomy. It was adopted by the International Astronomical Union in 1976 via
+    ///     Resolution No. 1, and has been significantly updated in 1994 and 2009.
     /// </summary>
     public partial class Astronomical : UnitSystem
     {
         /// <summary>
-        /// Construct a new instance of the CGS unit system
+        ///     Construct a new instance of the Astronomical unit system
         /// </summary>
         public Astronomical() : base(new Lazy<UnitSystemInfo[]>(GetDefaultSystemUnits))
         {