`pdal attribute`

It would be handy to have a kernel application that did the following:

* Define an OGR query to fetch polygons along with a column for values
* Select all the points inside each polygon in the OGR selection
* Set the attribute for the specified dimension to the result of the query

```
<?xml version="1.0" encoding="utf-8"?>
<Pipeline version="1.0">
    <Writer type="drivers.las.writer">
        <Option name="filename">
            pdal-compressed.laz
        </Option>
        <Filter type="filters.attribute">
            <Option name="query">
                SELECT GEOMETRY, ID FROM polygons
            </Option>
            <Option name="dimension">
                Classification
            </Option>
            <Reader type="drivers.las.reader">
                <Option name="filename">
                    ../las/1.2-with-color.las
                </Option>             
            </Reader>
        </Filter>
    </Writer>
</Pipeline>
```