[pdal] PdalXML style

[Michael P. Gerlek]

Here's what is now committed.  It is entirely read using
boost::property_tree, so no explicit XML programming is needed on our code
-- and, better still, we can use the ptree serializer to construct pipelines
using any supported notation, e.g. JSON.

-mpg


<?xml version="1.0"?>
<Pipeline>
    <Writer>
        <Type>drivers.las.writer</Type>
        <Option>
            <Name>filename</Name>
            <Value>out.las</Value>
            <Description>junk junk junk</Description>
        </Option>
        <Filter>
            <Type>filters.crop</Type>
            <Option>
                <Name>bounds</Name>
                <Value>([0,1],[0,1],[0,1])</Value>
            </Option>
            <Reader>
                <Type>drivers.las.reader</Type>
                <Option>
                    <Name>filename</Name>
                    <Value>../../test/data/1.2-with-color.las</Value>
                </option>
            </Reader>
        </Filter>
    </Writer>
</Pipeline>