ENH: allow astype conversions for timedeltas to other timedelta freqs (still returns a float series), related to GH4521

related #4521

```
In [4]: td = pd.to_timedelta([0,1,2],unit='D')

In [5]: td
Out[5]: 
0           00:00:00
1   1 days, 00:00:00
2   2 days, 00:00:00
dtype: timedelta64[ns]

In [6]: td / np.timedelta64(1,'s')
Out[6]: 
0         0
1     86400
2    172800
dtype: float64
```

This is new (used to raise)
```
In [7]: td.astype('timedelta64[s]')
Out[7]: 
0         0
1     86400
2    172800
dtype: float64
```