Miscellaneous Macros

When extending Coin, there are a number of macros one needs to know about and use.

When extending Coin, there are a number of macros one needs to know about and use.

### Node Macros

- SO_NODE_HEADER
- SO_NODE_ABSTRACT_HEADER
- SO_NODE_SOURCE
- SO_NODE_ABSTRACT_SOURCE
- SO_NODE_INIT_CLASS
- SO_NODE_INIT_ABSTRACT_CLASS
- SO_NODE_IS_FIRST_INSTANCE
- SO_NODE_CONSTRUCTOR
- SO_NODE_ADD_FIELD
- SO_NODE_DEFINE_ENUM_VALUE