`UFuncTypeError` when calling `summary` method

## Problem
When calling `result.summary()` we get:

```
UFuncTypeError: ufunc 'absolute' did not contain a loop with signature matching types <class 'numpy.dtypes.StrDType'> -> None
```

## `pymc_experiment` classes affected:
* `DifferenceInDifferences`

## TODO
- [ ] Fix this error
- [ ] If not done already, we should add test coverage for the `.summary` method