specify mods via JSON

This is necessary to translate modifications of the standard tax model into actual changes in a Calculator object. User interaction with the webapp (i.e filling in the data fields) will create a JSON string. This string will be passed to the `calculator` function to modify the Calculator.

Example JSON:
```
    {
        "_aged" : [[1500], [1200]]
    }
```

Use in code:
```
    >>>calc = calculator(user_json)
    >>>calc._aged
    array([[1500],
       [1200]])
```

cc @theofanislekkas @MattHJensen