Improvements and expansion of the introduction chapter.
These changes are in response to the email by Johnny Croy at
https://mail.gna.org/public/relax-users/2008-04/msg00028.html (Message-id:
<4E4F6DB4-5F27-4447-9EC9-6D2E9973A539@...>). The thread continues at
https://mail.gna.org/public/relax-users/2008-05/msg00000.html (Message-id:
<50dac1510bcb.510bcb50dac1@...>).
Most of this text comes from the relax website (http://nmr-relax.com).
