add other spectra described in ISO 18431-4

[CrepeGoat]

ISO 18431-4 describes various types of shock spectra, including:

• the absolute acceleration response spectrum (SRS)
• the pseudo-velocity response spectrum (PVSS)

While we currently implement the above two spectra, we do not implement the other spectra described in the ISO, like:

• the relative-velocity response spectrum
• the relative-displacement response spectrum
• the relative-displacement response spectrum expressed as equivalent static acceleration