Rework constructors to remove the Create pattern.

PkmnLibSharp/Library/GrowthRates/ExternGrowthRate.cs

@@ -13,7 +13,7 @@ namespace PkmnLibSharp.Library.GrowthRates
         {
         }
 
-        public static ExternGrowthRate Create(CalculateLevelDelegate levelFunc,
+        public ExternGrowthRate(CalculateLevelDelegate levelFunc,
             CalculateExperienceDelegate experienceFunc)
         {
             var ptr = IntPtr.Zero;
@@ -21,7 +21,7 @@ namespace PkmnLibSharp.Library.GrowthRates
                 Marshal.GetFunctionPointerForDelegate(levelFunc),
                 Marshal.GetFunctionPointerForDelegate(experienceFunc)
             );
-            return new ExternGrowthRate(ptr);
+            Initialize(ptr);
         }
 
         protected override void DeletePtr()

PkmnLibSharp/Library/GrowthRates/GrowthRate.cs

@@ -8,6 +8,8 @@ namespace PkmnLibSharp.Library.GrowthRates
         internal GrowthRate(IntPtr ptr) : base(ptr)
         {
         }
+        
+        protected GrowthRate(){}
 
         public byte CalculateLevel(uint experience)
         {