In a pipeline, infer the reader type from filename in the absence of a type attr

These two commits allow PDAL to infer the reader type from the reader's filename, instead of having to explicitly find the driver type in the pipeline.

In addition to beefing up a bit of logic in the `PipelineReader`, this pull request also adds a simpler `StageFactory::inferReaderDriver` static method that does not require any options as a second argument. These options were not used as part of the inference process, so I added a simpler method that just takes the filename and returns the appropriate driver type, if found. The old method has been updated to delegate the inference to this new method.