Add support for repeating tasks

[501st-alpha1]

As currently implemented, triggering a velocity calculation on a repeating task will overwrite the velocity every time, which isn't what I want. Instead, it should add the velocity calculation to the LOGBOOK, maybe as a line like this:

- Velocity: 1.382038

Might also set properties such as Last_Velocity and/or Average_Velocity.

Optional: have some way to calculate older velocities (though inserting in middle of LOGBOOK may be bad).