Deserialization for Kernel

[Mec-iS]

Kernel is now an object-safe trait so we can use its type as &dyn Kernel, it has also a name() function to have a string representation of the kernel as LinearKernel, RBFKernel, ...

We would like to provide a functionality that instantiate the kernel's default from a string:

"LinearKernel" -> LinearKernel::Default()

via a enum

pub enum KernelName {
    Linear(LinearKernel),
   ...
}

This possibility is known to be not possible in vanilla Rust but there is a library that provides it, see blogpost.

This feature should be only available and loaded in pairing with the "serde" feature so to avoid adding another library to the deps.

This will allow parameters that contain pub kernel: Option<&'a dyn Kernel<'a>>, to be deserializable.

[morenol]

I can take this, I think that we can get ride of name in Kernel trait, and instead handle that in the enum using:
#[serde(tag = "kernel_variant")]

[Mec-iS]

ok, the problem with Rust comes when is needed to go string->instance. there is no easy way like in Python getattr(module, string) to point to a class/type from a string/enum key.