Object hooks and isinstance tests

In `model/source.py` there are a few JSON decoder hooks specified. These currently perform `isinstance` checks against `dict` (e.g., in [`human_decoder`](https://github.com/biocore/microsetta-private-api/blob/master/model/source.py#L5)). Are these tests necessary? In the example below, the hook only appears to be used if the item coming in is a JSON object represented already as a `dict`:

```python
$ ipython
Python 3.6.7 | packaged by conda-forge | (default, Feb 28 2019, 02:16:08)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.9.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import json

In [2]: def hook(item):
...: print(type(item))
...: return item
...:

In [3]: json.loads('{"foo": "bar"}', object_hook=hook)

Out[3]: {'foo': 'bar'}

In [4]: json.loads('"foo"', object_hook=hook)
Out[4]: 'foo'

In [5]: json.loads('["foo", "bar"]', object_hook=hook)
Out[5]: ['foo', 'bar']
```