Implemented Softplus as a metaoperator

This Merge Request is related to this issue Issue335!

Instead of implementing Softplus operator as a standalone operator, it was suggested to implement it as a meta operator.

                                    softplus(x)=max(x,0)+log(1+e−∣x∣)

In order to implement Softplus as a meta operator, we also had to add these operators in aidge_core module

• Max
• CastLike