enable models (hdf5 files) to be independent of --max-len-per-line and --max-lines-per-commit

[asah]

Currently, saved models (.hdf5 files) are hard-coded to a given --max-len-per-line and --max-lines-per-commit setting, because these values are embedded in the tensor-shapes in the neural net architecture.

Subtly, we can't just use a large value for these settings because of GPU RAM requirements.

This issue will "go away" in <1 year as GPU hardware radically improves.