Feature: add parameters via method chaining

[dynobo]

The current way to construct a ParameterSpace is

space = ParameterSpace()
space = space.add(...)
space = space.add(...)
space = space.add(...)
...

This feels a bit verbose, and method chaining seems a bit more convenient:

space = (
    ParameterSpace()
    .add(...)
    .add(...)
    .add(...)
    ...
)

This PR gives the user to choose between both syntaxes.

[LGro]

I usually am a fan of there is one way to do things, but I have also wondered about allowing that to avoid copying the object instance variable over and over. I'd still wait for feedback from @sfalkner before moving forward, but you have my approval ☺️

[sfalkner]

Simple change, and if it improves the user experience for some of us, why not. Thanks @dynobo !