Python extension improvements

[hobu]

• pdal.Pipeline class with schema, log, pipeline, and arrays properties
• Python extension calls play nicely with embedded Python filters
• PipelineExecutor helper class

[hobu]

Basic usage such as this now supported:

  json = """
    {
      "pipeline": [
        "1.2-with-color.las",
        {
            "type": "filters.sort",
            "dimension": "X"
        }
      ]
    }"""

    import pdal
    pipeline = pdal.Pipeline(pipeline)
    pipeline.loglevel = 9 #really noisy
    count = pipeline.execute()
    arrays = pipeline.arrays
    metadata = r.metadata
    log = pipeline.log