Add setting precision for json writers and also add decimal places precision type. (#752)

* Added setting precision for writers.
* Added special case for precise precision and global precision.
* Added good setting of type of precision and also added this type to BuiltStreamWriter and for its settings.
* Added some tests.

include/json/value.h

@@ -109,6 +109,13 @@ enum CommentPlacement {
   numberOfCommentPlacement
 };
 
+/** \brief Type of precision for formatting of real values.
+ */
+enum PrecisionType {
+  significantDigits = 0, ///< we set max number of significant digits in string
+  decimalPlaces          ///< we set max number of digits after "." in string
+};
+
 //# ifdef JSON_USE_CPPTL
 //   typedef CppTL::AnyEnumerator<const char *> EnumMemberNames;
 //   typedef CppTL::AnyEnumerator<const Value &> EnumValues;
@@ -220,6 +227,9 @@ public:
   static const UInt64 maxUInt64;
 #endif // defined(JSON_HAS_INT64)
 
+  /// Default precision for real value for string representation.
+  static const UInt defaultRealPrecision;
+
 // Workaround for bug in the NVIDIAs CUDA 9.1 nvcc compiler
 // when using gcc and clang backend compilers.  CZString
 // cannot be defined as private.  See issue #486