Add basic pipeline tools

Motivation

The same code is repeated in many places for creating a pipeline, coercing strings to Caps, and running a pipeline. This MR gathers these into a single place.

Changes

• added several utilities to gstlal.gsttools for creating and executing gstreamer pipelines
• renames gstlal.gsttools to a more specific gstlal.gstpipetools

Related Issues

Closes: #88 (closed) Related: #90, #91

Examples

Using the new utilities it's possible to put together a simple GStreamer pipeline and run.

# Import utilities
from gstlal import gsttools
from gstlal.pipeparts import source, sink, transform, encode 

# Construct the pipeline
pipeline = gsttools.make_pipeline('Tutorial101')
head = source.audio_test(pipeline, wave=source.AudioTestWaveform.Sine)
head = transform.amplify(pipeline, head, amplification=0.5)
head = encode.wav(pipeline, head)
head = sink.file(pipeline, head, 'sample-out.wav')

# Execute the pipeline
gsttools.run_pipeline(pipeline, segment=(0, 1))

requested review from @patrick.godwin

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