lalapps_psinject: test suite, port to LALFrame, documentation

Some items to address for the CW hardware injection code lalapps_psinject:

• This code should have a test suite, to check that the way the code is used in production doesn't break
• Parts of the code optionally write out frames using FrameL directly. If this code is still needed (to check) this should be ported to LALFrame, so that either FrameL or FrameCPP can be used, and a direct dependency on FrameL can be avoided
• Is there any better documentation for this code? At the moment there's only a very old README dating from 2003 (S3!)