Theoretically ready for general use

Made datum adjustment optional. I believe this was the last critical barrier to general use; all remaining points (e.g. multi-flow-range calibration) are convenience features.

The attached Demo.zip includes everything needed to try out Raspy-Cal: an example project, a stage file, and configuration files for use with the command line and importing into the GUI. The stage file is based on the results from the example project with variations added to simulate real data that will not perfectly fit Manning's Equation.