refactor: use invoke for predict calls and replace do.call for invoke where relevant

[m-muecke]

Changes:

• Always use param_set$get_values() to access param set values
• Use warningf() instead of custom warning condition
• Set dbscan and co to density property instead of partioning
• Always use invoke() in train and predict instead of do.call()
• Use checkmate convention for naming of function names in custom assertions
• Simplify the parameter checks where possible

[damirpolat]

Looks good to me. Thank you for your PR!