Add more content to "Partial installation"

[jypeter]

Thanks @davis278 for what you did in #157!

In the Partial installation section:

• could you specify if cdutil/genutil/cdtime (and other PCMDI packages) can be installed separately, or if they are bundled with cdms2 and/or vcs
• it could be useful to mention how to install cmor along with a link to CMOR3 site
• it would be useful to add a link to the documentation page of each package