This PR adds numeric scalar conversions, such as float to int, inside the basic_node::get_value function.

fkyaml::node n = 1.23; // type == node_type::FLOAT
int i = n.get_value<int>(); // i == 1 (`double` is internally converted to `int`)
bool b = n.get_value<bool>(); // b == true (`double` is internally converted to `bool`)

This change was suggested here by @ARessegetesStery to provide easier and more natural access methods to YAML node values in a similar way in which C++ handles primitives.

The conversions follows these rules:

• If the YAML node value is a null (node_type::NULL_OBJECT), the value can be converted to:
  • false (boolean)
  • 0 (integer)
  • 0.0 (floating point)
• If the YAML node value is a boolean (node_type::BOOLEAN), the value can be converted to:
  • 1 /*true*/ or 0 /*false*/ (integer)
  • 1.0 /*true*/ or 0.0 /*false*/ (floating point)
• If the YAML node value is a integer (node_type::INTEGER), the value can be converted to:
  • true /*non-0*/ or false /*0*/ (boolean)
  • static_casted floating point value (floating point)
• If the YAML node value is a floating point (node_type::FLOAT), the value can be converted to:
  • true /*non-0*/ or false /*0*/ (boolean)
  • static_casted integer value (integer)

If all the above are not applied, the basic_node::get_value function throws a type_error exception, which is unchanged.

Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
• The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.