Change default float and double values to minimum representable values.

Previously, we were using the minimum positive value as the `minValue`.

Author: ZachBray

sbe-tool/src/main/java/uk/co/real_logic/sbe/PrimitiveValue.java

@@ -197,7 +197,7 @@ public enum Representation
     /**
      * Maximum value representation for a single precision 32-bit floating point type.
      */
-    public static final float MIN_VALUE_FLOAT = Float.MIN_VALUE;
+    public static final float MIN_VALUE_FLOAT = -Float.MAX_VALUE;
 
     /**
      * Maximum value representation for a single precision 32-bit floating point type.
@@ -212,7 +212,7 @@ public enum Representation
     /**
      * Minimum value representation for a double precision 64-bit floating point type.
      */
-    public static final double MIN_VALUE_DOUBLE = Double.MIN_VALUE;
+    public static final double MIN_VALUE_DOUBLE = -Double.MAX_VALUE;
 
     /**
      * Maximum value representation for a double precision 64-bit floating point type.