Fix: Use &self argument for various unit methods, not self

If we use `self`, without the ampersand, then calling the method will consume the struct (like moving it) and it won't be usable again.

Author: alexdewar

src/units.rs

@@ -60,19 +60,19 @@ macro_rules! base_unit_struct {
         }
         impl $name {
             /// Returns the underlying f64 value.
-            pub fn value(self) -> f64 {
+            pub fn value(&self) -> f64 {
                 self.0
             }
             /// Returns true if the value is a normal number.
-            pub fn is_normal(self) -> bool {
+            pub fn is_normal(&self) -> bool {
                 self.0.is_normal()
             }
             /// Returns true if the value is finite.
-            pub fn is_finite(self) -> bool {
+            pub fn is_finite(&self) -> bool {
                 self.0.is_finite()
             }
             /// Returns the absolute value of this unit.
-            pub fn abs(self) -> Self {
+            pub fn abs(&self) -> Self {
                 $name(self.0.abs())
             }
         }