Add MathOp helper methods

It's helpful to be able to assert what type of Math error occurred.

units/src/result.rs

@@ -254,6 +254,18 @@ impl MathOp {
 
     /// Returns `true` if this operation error'ed due to division by zero.
     pub fn is_div_by_zero(self) -> bool { !self.is_overflow() }
+
+    /// Returns `true` if this operation error'ed due to addition.
+    pub fn is_addition(self) -> bool { self == MathOp::Add }
+
+    /// Returns `true` if this operation error'ed due to subtraction.
+    pub fn is_subtraction(self) -> bool { self == MathOp::Sub }
+
+    /// Returns `true` if this operation error'ed due to multiplication.
+    pub fn is_multiplication(self) -> bool { self == MathOp::Mul }
+
+    /// Returns `true` if this operation error'ed due to negation.
+    pub fn is_negation(self) -> bool { self == MathOp::Neg }
 }
 
 impl fmt::Display for MathOp {