ncgeodataset: fix geodataset{name}(indexes).data method and add "help ncgeodata...

...set" examples

I added some examples of ncgeodataset methods.  I ran into an issue with using the .data method in the subsref stuff which took a small patch.

It also looks like the gettimedata, getlatdata, getlondata functions in ncgeodatast.m may not be usable.