[CF-metadata] CF point observation Conventions ready for review

[John Caron]

I have complete a new version of the CF point observation Conventions at:

  https://cf-pcmdi.llnl.gov/trac/wiki/PointObservationConventions

Discussion is at:

  https://cf-pcmdi.llnl.gov/trac/ticket/37

I have incorporated various feedback from the past year, and made a 
preliminary implementation to be sure that a generic application can 
distinguish the various cases without human intervention.

I have tried to simplify, esp in the station profile and section feature 
types, where the combinations of options got too complex. The document 
is now explicit about all possible representations. The use of missing 
values is also clarified.

While the document is rather long, if you manage to wade through it 
you'll see that the patterns of use keep repeating and are mostly 
regular. Showing full examples I think is the best way to prevent 
misunderstandings.

I did make enough changes that anyone who wrote files using the previous 
version should check to see whats changed. Apologies for that.