IQuantity: Change IDivisionOperators, add Zero property

- Add `Zero` static abstract property, already implemented by all quantities.
- Remove redundant `IEqualityOperators` constraint, covered by `IComparisonOperators`.
- Change `IDivisionOperators` to divide by number instead of TSelf, needed for average calculation.

Got error trying to keep both IDivisionOperators declarations:
```
Interface 'UnitsNet.IArithmeticQuantity<TSelf,TUnitType,TValueType>' cannot implement both 'System.Numerics.IDivisionOperators<TSelf,TValueType,TSelf>' and 'System.Numerics.IDivisionOperators<TSelf,TSelf,TValueType>' because they may unify for some type parameter substitutions
```

Author: angularsen

UnitsNet/IQuantity.cs

@@ -3,7 +3,9 @@
 
 using System;
 using System.Globalization;
+#if NET7_0_OR_GREATER
 using System.Numerics;
+#endif
 using UnitsNet.Units;
 
 namespace UnitsNet
@@ -123,7 +125,6 @@ public interface IQuantity<TUnitType> : IQuantity where TUnitType : Enum
     /// <typeparam name="TValueType">The underlying value type for internal representation.</typeparam>
     public interface IQuantity<TSelf, TUnitType, TValueType> : IQuantity<TUnitType>
 #if NET7_0_OR_GREATER
-        , IEqualityOperators<TSelf, TSelf, bool>
         , IComparisonOperators<TSelf, TSelf, bool>
         , IParsable<TSelf>
 #endif
@@ -145,12 +146,21 @@ public interface IArithmeticQuantity<TSelf, TUnitType, TValueType> : IQuantity<T
         , IAdditiveIdentity<TSelf, TSelf>
         , ISubtractionOperators<TSelf, TSelf, TSelf>
         , IMultiplyOperators<TSelf, TValueType, TSelf>
-        , IDivisionOperators<TSelf, TSelf, TValueType>
+        , IDivisionOperators<TSelf, TValueType, TSelf>
         , IUnaryNegationOperators<TSelf, TSelf>
 #endif
         where TSelf : IArithmeticQuantity<TSelf, TUnitType, TValueType>
         where TUnitType : Enum
         where TValueType : struct
+#if NET7_0_OR_GREATER
+        , INumber<TValueType>
+#endif
     {
+#if NET7_0_OR_GREATER
+        /// <summary>
+        ///     The zero value of this quantity.
+        /// </summary>
+        static abstract TSelf Zero { get; }
+#endif
     }
 }