Added Gigameter (1e+9 meters) (#1318)

Gigameter is really useful for my case, even though it is a bit obscure,
when rendering astronomical distances it seems to hit the perfect spot.
While I use AU and meter during the calculations, these intermediate
units really help when it comes to rendering the simulation itself. It
seems to be a good fit for making sure that both short and long stellar
distances can be rendered without too much trouble.

I am completely fine with this getting rejected as the number of meter
units this library is probably getting out of hand, but I wanted to
create a PR to see if it would be a good fit for the library.

I followed the contribution page and also checked out the PR for
Megameters (#1208) but please let me know if I missed something.

Author: yigitlevent

Common/UnitDefinitions/Length.json

@@ -14,7 +14,7 @@
       },
       "FromUnitToBaseFunc": "{x}",
       "FromBaseToUnitFunc": "{x}",
-      "Prefixes": [ "Femto", "Pico", "Nano", "Micro", "Milli", "Centi", "Deci", "Deca", "Hecto", "Kilo", "Mega" ],
+      "Prefixes": [ "Femto", "Pico", "Nano", "Micro", "Milli", "Centi", "Deci", "Deca", "Hecto", "Kilo", "Mega", "Giga" ],
       "Localization": [
         {
           "Culture": "en-US",

Common/UnitEnumValues.g.json

@@ -695,7 +695,8 @@
     "Megameter": 41,
     "Kilofoot": 45,
     "Femtometer": 48,
-    "Picometer": 43
+    "Picometer": 43,
+    "Gigameter": 44
   },
   "Level": {
     "Decibel": 1,

UnitsNet.NanoFramework/GeneratedCode/Quantities/Length.g.cs

@@ -86,6 +86,7 @@ public class LengthTests : LengthTestsBase
         protected override double DataMilesInOneMeter => 0.000546807;
 
         protected override double MegametersInOneMeter => 1e-6;
+        protected override double GigametersInOneMeter => 1e-9;
 
         protected override double KilofeetInOneMeter => 3.28083989501e-3;
 

UnitsNet.NanoFramework/GeneratedCode/Units/LengthUnit.g.cs

@@ -10,6 +10,7 @@ DtpPoints=pt
 Fathoms=fathom
 Femtometers=fm
 Feet=ft,',′
+Gigameters=Gm
 Hands=h,hh
 Hectometers=hm
 Inches=in,\",″