Create zero.go

Zero.go defines convenience functions for testing decimal values with the zero value. These functions make code using decimal easier to read in my opinion. EqualZero() could be renamed into IsZero()

zero.go

@@ -0,0 +1,31 @@
+package decimal
+
+// EqualZero returns whether the numbers represented by d equals zero.
+func (d Decimal) EqualZero() bool {
+	return d.Equal(Zero)
+}
+
+// NotZero returns whether d is not zero
+func (d Decimal) NotZero() bool {
+	return !d.EqualZero()
+}
+
+// GreaterThanZero (GT0) returns true when d is greater than zero.
+func (d Decimal) GreaterThanZero() bool {
+	return d.GreaterThan(Zero)
+}
+
+// GreaterThanOrEqualZero (GTE0) returns true when d is greater than or equal to zero.
+func (d Decimal) GreaterThanOrEqualZero() bool {
+	return d.GreaterThanOrEqual(Zero)
+}
+
+// LessThanZero returns true when d is less than zero.
+func (d Decimal) LessThanZero() bool {
+	return d.LessThan(Zero)
+}
+
+// LessThanOrEqualZero returns true when d is less than or equal to zero.
+func (d Decimal) LessThanOrEqualZero() bool {
+	return d.LessThanOrEqual(Zero)
+}