Add "a" parameter to softplus

[DominiqueMakowski]

The "a" parameter can control the smooth function, which is useful in the context of regression modelling.

• See https://link.springer.com/article/10.1007/s00362-023-01509-x

Would it be possible to add it as an optional argument (defaulting to 1) ?

[tpapp]

Fixed by #85.