Improve EMR advanced current calibration

[csatt]

One of the main advantages of the SSR and FET designs over the EMR design has been the ability to accurately calibrate the current measurements. Current measurement calibration for the EMR design is very cumbersome (requiring an external switch and a clear sunny day.)

However, hiding in plain sight has been a much better way. In fact, this method works with the current software if the user "lies" about which type of relay they have on the advanced current calibration dialog. It requires a jumper wire with an alligator clip on one end. This jumper is connected between the red binding post (PV+) and the exposed leg of the bleed resistor (Rb). The exposed leg of Rb is the LOAD_CAP- signal, which is also connected to the top of the shunt resistor. That creates a path between PV+ and PV- that bypasses the load capacitors but goes through the shunt. It is safe for this path to carry 10A of current (or more) indefinitely. As with the SSR and FET designs, this makes it possible to accurately calibrate an EMR-based IVS2 with a bench power supply (no sun required.)

The software should be changed so the user doesn't need to "lie" about which type of relay they have. When the relay type is EMR on the advanced current calibration dialog, the actions should be the same as they are when the SSR/FET relay type is selected, except the "cooling off" period should not be enforced. The help dialog should be changed to describe the new methodology.