Add possibility of user defined bandwidth in rdrobust

[OliverSchacht]

Description

Add the possibility to set "h" and "b" as kwargs in RDFlex

Reference to Issues or PRs

Requested in #341

Comments

Not recommended to set because of iterative bandwidth selection in RDFlex, warning is included.

PR Checklist

• [x] The title of the pull request summarizes the changes made.
• [x] The PR contains a detailed description of all changes and additions.
• [x] References to related issues or PRs are added.
• [] The code passes all (unit) tests.
• [] Enhancements or new feature are equipped with unit tests.
• [x] The changes adhere to the PEP8 standards.