[pdal] API dox examples

[Howard Butler]

Michael,

Here's what some of the output looks like using the setup I proposed yesterday:

http://pointcloud.org/api/cpp/dimension.html
http://pointcloud.org/api/cpp/bounds.html

Once a class has been rendered through the sphinx system as above, we can now link to it in documentation like:

:cpp:class:`pdal::Dimension`
:cpp:class:`pdal::Bounds`

Now to write class documentation... :)

Howard