Calculates overlapping frequency between f1 and f2.
Or, put more accurately, this returns a Frequency that is the part of f1 that is overlapped by f2. The resultant start frequency is the smallest f1.f that is greater than f2.f.start, and the stop frequency is the largest f1.f that is smaller than f2.f.stop.
This way the new frequency overlays onto f1.
f3 – part of f1 that is overlapped by f2