pub fn as_types(&mut self, dtypes: Vec<DType>) {
        assert_eq!(
            self.data.len(),
            dtypes.len(),
            "Length of dtypes are not compatible with DataFrame"
        );
        for (i, dtype) in dtypes.into_iter().enumerate() {
            self[i].as_type(dtype);
        }
    }
Type casting for DataFrame

# Examples

```rust
extern crate peroxide;
use peroxide::fuga::*;

fn main() {
let a = Series::new(vec![1i32, 2, 3, 4]);
let b = Series::new(vec![true, false, false, true]);

let mut df = DataFrame::new(vec![a, b]);    // I32, Bool
df.as_types(vec![USIZE, U8]);               // USIZE, U8

let c = Series::new(vec![1usize, 2, 3, 4]);
let d = Series::new(vec![1u8, 0, 0, 1]);
let dg = DataFrame::new(vec![c, d]);

assert_eq!(df, dg);
}
```
src/structure/dataframe.rs