text: Title: [FEATURE-REQUEST] Getting dtype of columns as they are when rendered in a pandas dataframe?
  Body: **Description**
  Hello, I would like to get the dtype of the columns as they are when the vaex dataframe is turned into a pandas dataframe.
  Basically, vaex is using its own dtype.
  ```python
  import vaex
  df=vaex.from_items(("a",[1,2,3]),("b",[1.1, 2.1, 3.1]))
  type(df.dtypes['a'])
  Out[64]: vaex.datatype.DataType
  ```
  But,
  ```python
  type(vf[:1].to_pandas_df().dtypes.to_dict()['a'])
  Out[65]: numpy.dtype[int64]
  ```
  
  Please, is there any way to get the result of the 2nd method without having vaex to compute a row? (in example above, I am making vaex computing the 1st row) If it is possible, I would like to prevent it, because I am using this information in a setup step.
  
  Thanks for your help!