Added more comments/documentation only

Author: Martin D. Weinberg

expui/UnitValidator.H

@@ -6,6 +6,29 @@
     can be extended as needed by adding new names and aliases to
     UnitValidator::createAllowedUnitTypes() and
     UnitValidator::createAllowedUnitNames() in UnitValidator.cc
+
+    Strategy:
+    -------
+    - Types and their aliases are stored in an std::unordered_map
+    - Units and their aliases are stored in an std::map of
+      std::unordered_maps
+    - The operator()(type, unit, value) first checks if the provided
+      type exists in the allowed types map.  If it does, it retrieves
+      the canonical type string which is used to look up the allowed
+      units for that type in the std::map.  Then, the unit type is
+      checked against the allowed units for that type.  If both type
+      and unit are valid, the canonical strings are returned.
+
+    Usage:
+    -----
+    - Create an instance of UnitValidator.  Currently, this is done in
+      the Coefficients class by default.
+    - Call the operator() on an instance with the type and unit
+      strings to be validated
+    - The operator() returns a tuple with:
+       + a boolean indicating if the type and unit are valid
+       + the canonical type string, or "unknown" if invalid
+       + the canonical unit string, or "unknown" if invalid
 */
 class UnitValidator
 {