[pdal] XYZ reader

[Michael P. Gerlek]

A thought on the XZY reader driver...

The xyz text file will want to be accompanied by a metadata file which gives the name and data type of each of the text columns (Dimensions). This could be done using simple text:

	X float64
	Y float64
	Z uint16
	T uint64

But this is really just a serialization of the Schema. And we might also want to be able to push other metadata about that xyz file into the pipeline, so maybe we could use json to hold the schema, the coordinate system, and any arbitrary key-value pairs the user might want to push in there.

-mpg