Sixteen channel shield update

I've made an update to the sixteen channel shield, taking it from ARD-SRG-IPS4X4-1 to ARD-SRG-IPS4X4-2. The new boards started going out a couple of weeks ago, and they should be reaching customers now. The most important change for end users is that the DB-25 connector now comes loose rather than soldered down. There are two other changes, made to ease manufacture and inventory. The protection diodes were rotated and replaced with another member of the same series with less critical orientation requirements. The eight-gang resistors were replaced with four-gang resistors. I'm now out of stock of the -1 variant, so all orders will receive RoHS compatible -2 boards from now on.