Modify inject signal

This restructures some functionality surrounding the inject_signal method.

• Added inject_signal_from_waveform_generator and inject_signal_from_waveform_polarizations that are slightly less confusing to use than inject_signal.
• Removed if/else statement that checks size of the injected signal. The injection will now just fail if that condition is now not fulfilled. It was already breaking before, but at a different place.
• Added explicit tests for these methods.