Issue #14. Comment that empty string is ok

src/main/java/org/djunits/unit/Unit.java

@@ -1048,7 +1048,8 @@ public String toString()
     }
 
     /**
-     * Find or create a unit for the given SI dimensions.
+     * Find or create a unit for the given SI dimensions. Note that unitString may be empty, corresponding to a Dimensionless
+     * unit.
      * @param unitString String; the textual representation of the unit
      * @return SIUnit; the unit
      * @throws IllegalArgumentException when the unit cannot be parsed or is incorrect
@@ -1057,7 +1058,6 @@ public String toString()
     public static SIUnit getUnit(final String unitString)
     {
         Throw.whenNull(unitString, "Error parsing SIVector: unitString is null");
-        Throw.when(unitString.length() == 0, IllegalArgumentException.class, "Error parsing SIVector: empty unitString");
         try
         {
             SIUnit unit = Unit.lookupOrCreateUnitWithSIDimensions(SIDimensions.of(unitString));

src/main/java/org/djunits/unit/si/SIDimensions.java

@@ -154,7 +154,7 @@ public static SIDimensions of(final String siString) throws UnitException
      * Translate a string representing SI dimensions to an SIDimensions object. Example: SIDimensions.of("kgm2") is translated
      * to a vector {0,0,1,2,0,0,0,0,0}. It is allowed to use 0 or 1 for the dimensions. Dimensions between -9 and 9 are allowed.
      * The parsing is quite lenient: periods and carets (^) are taken out, and the order can be arbitrary, so "kgms-2" is
-     * accepted as well as "m.s^-2.kg"
+     * accepted as well as "m.s^-2.kg". Note that the empty string parses to the dimensionless unit.
      * @param siString String; concatenation of SI units with positive or negative dimensions. No divisions sign is allowed.
      * @return byte[]; a vector of length <code>NUMBER_DIMENSIONS</code> with the dimensions for the SI units
      * @throws UnitException when the String cannot be parsed, e.g. due to units not being recognized