draft: ENH: Add InjectionSource element
This implements the InjectionSource element, which reads from an injection hdf to stream waveform data. A separate transform element will be needed to add the waveform data into the general data stream.
Currently a given source produces strain from a single detector, writing to a single source pad. This uses an internal buffer to limit the number of waveform recomputations, and the number of waveforms held in memory at a given time.
Most docstrings are complete, tests are not yet written.