Need way to specify starting source well

[mkapasiawala]

Currently, the well of a source plate that is the starting well is obtained from the .dat file. In the absence of an existing .dat file, a new one is created and the source wells start at A1. It would be nice to add functionality that could override this when desired, by specifying a starting well as an argument. The existing implementation of skipping one wells between different source materials is good.

We could also implement this by having the user specify all source wells, but this is more error-prone.