The mimeEntity object contains a MIME entity. It can be
created (obtained) from a mimeReader object at its current
cursor position. There are access functions for obtaining information
about the entity, such as headers, body, etc.
Since these access functions also work on mimeReader objects,
typically they are used directly on the mimeReader for
convenience, without ever explicitly creating a mimeEntity.
However, if an entity or entity tree of a message must be "copied
off" for separate handling elsewhere while the rest of the message
continues to be parsed, a mimeEntity can be created at that
point to contain such an entity. A new mimeReader could then
be created to parse that separated entity.