ENH: Set numeric_only=True in aggregations
I've noticed that I make this mistake quite often:

```python
>>> df = pd.DataFrame({"a": ["1"] * 3, "b": np.ones(3)})
>>> df.sum()
a    111
b      3
dtype: object
```

Getting `111` as a result in column a is harmless in this example, but actually quite annoying in most real life use cases where it can produce exceedingly large strings that exhaust memory or tie up the interpreter.

The `numeric_only` argument gets around this issue:

```python
>>> df.sum(numeric_only=True)
b    3.0
dtype: float64
```

Though I'm curious if this should really be the default
