text: Title: Setting `ids` in `@parametrize` leads to "ValueError: Only one of ids and idgen should be provided"
  Body: Using `ids` without setting `idgen` to None explicitly leads to this error.
  
  ```python
  from pytest_cases import parametrize, parametrize_with_cases
  
  
  class Person:
      def __init__(self, name):
          self.name = name
  
  
  def get_tasks():
      return [Person("joe"), Person("ana")]
  
  
  class CasesFoo:
      @parametrize(task=get_tasks(), ids=lambda task: task.name)
      def case_task(self, task):
          return task
  
  
  @parametrize_with_cases("task", cases=CasesFoo)
  def test_foo(task):
      print(task)
  ```
  
  A workaround is to set `idgen=None` too: `@parametrize(task=get_tasks(), ids=lambda task: task.name, idgen=None)`
  
  See also #237