Add generationCode to Germplasm

[daveneti]

The generation code is a simple code that denotes the generation since the last generative cross. There are several types of code generally used, although it is up to the implementing systems to define these.

For simple crosses, F1 is use for the progeny of the cross, F2 for selfed crosses of these progeny, F3 for next generation etc.

For backcrossing BC is used, with BC1 bring used for offspring resulting from the first cross back to the recurrent parent in the backcross breeding method. Then BC2 would the case when the BC1 generation is crossed to the recurrent parent gain.

When the backcross progeny are selfed you would use a combination of F and BC. For example BC1-F3, would be progeny from a single round of back crossing followed by 3 rounds of selfling.

Other breeding methods other codes could be used. For example M for mutations, and subsequent self crosses.