Add mixing events to the library method

[deepanshs]

Add mixing event to 2D library methods.

[mgiammar]

@pjgrandinetti @deepanshs This issue relates to the fact that library 2D methods use the fictitious total mixing between events (pathways all have amplitude of 1) and that total mixing behavior might lead to discrepancies in spectral amplitudes.

Should we work towards adding in explicit MixingEvent objects between events for named methods (e.g. angle of 0 and phase of 0)?