Create the first model generator

[raar1]

For this issue we will use the transformer retention time model generating code already available in the package here

• Add a modelgen directory to the repo (that will be accessible in the python package as aiproteomics.modelgen) to put any new model generator code in
• Adapt the code so that it outputs the model in .onnx format (allow optionally still to output the model as a keras model as it currently does)
• Make the file optionally also runnable from the commandline (using the if __name__ == "__main__": approach). When run from the commandline, it should only output in ONNX format