[pdal] Pipeline XML as primary channel?

[Michael P. Gerlek]

Howard suggests we use pipeline XML as the "preferred" means of working with
PDAL.

I'm in favor, but with these thoughts:

- cmd line apps will still need to have the ability to take regular files
and switches as input, because you don't want to make XML files for
one-off/simple things.  (Although maybe the cmd line args just get quickly
converted to an XML chunk and then passed along...)

- We might need some sort of "argument substitution" mechanism, so an
arbitrary XML pipeline can be written in "template" form without needing to
specify a filename.  That way, a pipeline can be "reused" across multiple
input files, for example.

- Should we migrate XML format to use attributes?

- XML format needs versioning mechanism

- XML pipelines can express a fully formed "writer" or open-ended "reader"
pipeline.  Ought to add ability to express "partial" pipelines, that don't
have a reader on one end or a writer on the other end.  Such pipeline
fragments can be used to represent a particular kind of processing step
("decimate, reproject, then colorize") that someone might want to be able to
reuse with other pipeline pieces.

Comments, thoughts?

-mpg