Add inch and feet as allowed units

src/MDUnitsSet.php

@@ -16,7 +16,9 @@ final class MDUnitsSet extends MDValueSet {
 
     const UNITS_LENGTH_TO_MILLIMETER = [
         'm'  => 1000,
+        'ft' => 304.8,
         'dm' => 100,
+        'in' => 25.4,
         'cm' => 10,
         'mm' => 1,
     ];