Rename abstract methods

[MalteKurz]

Description

In alignment with the Python package (see DoubleML/doubleml-for-py#131), in this PR the abstract methods are renamed:

• ml_nuisance_and_score_elements --> nuisance_est
• ml_nuisance_tuning --> nuisance_tuning

Comments

Blocked by #161 (derived from there to prevent merge conflicts).

PR Checklist

• The title of the pull request summarizes the changes made.
• The PR contains a detailed description of all changes and additions.
• The code passes R CMD check and all (unit) tests.
• The changes adhere to the "mlr-style" standards.