Add command-line argument to switch between PSD, ASD and time-series

Currently PSD is computed by default, and flag --time-series is used to switch to time-series plotting.

Users should be able to choose the scaling (PSD vs. ASD) or the time series quicklook option using a global switch or multiple options (still TBD). Note that subsequent options will not be available, or will have different meanings depending on the chosen type of computation (eg. --nperseg is only available for PSD and ASD estimation).