Non-minor rewrites of the design spec to accomodate new requirements

See the included commits for details. Includes many fixes for the preliminary release version.

src/SparseTransform/Convert/ErrorListener.cs

@@ -22,7 +22,7 @@ namespace Transform.Convert
         /// <exception cref="Exception"></exception>
         public void SyntaxError([NotNull] IRecognizer recognizer, [Nullable] IToken offendingSymbol, int line, int charPositionInLine, [NotNull] string msg, [Nullable] RecognitionException e)
         {
-            throw new Exception(msg);
+            throw new Exception("Error in MatrixMarket header line: " + msg);
         }
     }
 }