statnetStartupMessage: Construct a "standard" startup message to be printed when the package is
loaded.
Description
This function uses information returned by packageDescription()
to construct a standard package startup message according to the
policy of the Statnet Project.
A string containing the startup message, to be passed to the
packageStartupMessage() call or NULL, if policy
prescribes printing default startup message. (Thus, if
statnetStartupMessage() returns NULL, the calling package should
not call packageStartupMessage() at all.)