Remove `n_rows` and `levels_back` parameters

[HenrikMettler]

For historical reasons, our genome has parametersn_rows, levels_back. However they offer little benefit in optimization and in actual applications these are never used. To simplify things and improve the user experience we should remove these parameters.

[HenrikMettler]

One question that arises, is whether we should also stop using the name n_columns and simply name it n_hidden since these things would become equal

[jakobj]

yes, i think that would be a good idea. n_columns may be difficult to understand without the option of adding rows :D