✨ ChangeFormat fluent style returning itself

[pleonex]

Return the same node when calling ChangeFormat to allow method chaining.

This PR closes #196

Quality check list

• Related code has been tested automatically or manually
• Related documentation is updated
• I acknowledge I have read and filled this checklist and accept the
  developer certificate of origin

Acceptance criteria

• ChangeFormat returns the same node

Follow-up work

Further improvements will continue in #195 with an overload to import files.

Example

using var node = new Node("node")
    .ChangeFormat(binFormat)
    .TransformWith<Binary2Po>();