Return fixed length array with add_baseline_and_extend_tail

In the function [`add_baseline_and_extend_tail`](https://github.com/JuliaPhysics/SolidStateDetectors.jl/blob/e43fd4bcd3fb1e70a78651e505ac5312cc25884e/src/Event/Event.jl#L288-L305), the output padded waveforms are returned as Vectors. It would be nice to instead return as fixed-length Arrays, since they will all have the same length. This would be beneficial for future processing of waveforms since processing based on recorded data tends to assume a fixed-length waveform as determined by the DAQ. https://github.com/JuliaPhysics/SolidStateDetectors.jl/blob/e43fd4bcd3fb1e70a78651e505ac5312cc25884e/src/Event/Event.jl#L288-L305

Read src/Event/Event.jl lines 288-305 and inspect add_baseline_and_extend_tail's current return values. Check the existing test coverage around waveform extension, then verify that padded waveforms are returned with one consistent fixed length rather than as variable-length vectors.

Written by the indexing model from the issue text.