[pdal] Pipeline XML as primary channel?

[Michael P. Gerlek]

> `//Filter[@type="filters.reprojection"]` seems cleaner than
> `//Filter/Type[text()="filters.reprojection"/..`.

Wow, they both just look like so much @$#^@%$//! line noise to me... :-)

> * <opinion>Attributes work well for atomic values e.g. driver types and
> option names.</opinion>

<opinion assertedas="fact">Agreed.</opinion>

> <WriterPipeline>
>     <Writer type="drivers.oci.writer">
>         <Option name="debug">true</Option>
>         <Reader type="drivers.las.reader">
>             <Option name="filename">in.las</Option>
>         </Reader>
>     </Writer>
> </WriterPipeline>

Yup, that's basically what I had when I started the project -- but then I
found out that boost::property_tree doesn't serialize to XML attrs quite as
cleanly as I'd like.  I'm going to revisit that issue this afternoon, I
think.

-mpg