Custom JSON encoder for nan and inf

See #21 for a discussion. At some point we may want to protect against serializing `NaN` and `inf` in JSON returned from flask, so we could do something like create a custom JSON encoder that replaces these values with `null`:

```
import flask

def safe_json_encoder(replace_nan=None, replace_inf=None):
class JSONEncoder(flask.json.JSONEncoder):

def default(self, o):
# some function that replaces nan with `replace_nan` and inf with `replace_inf`

return JSONEncoder

app = # create flask app
app.json_encoder = safe_json_encoder()

```

For more details see:
* [Pass user built JSON Encoder into Flask's jsonify](https://stackoverflow.com/questions/44146087/pass-user-built-json-encoder-into-flasks-jsonify)
* [Flask JSONEncoder documentation](https://flask.palletsprojects.com/en/1.1.x/api/#flask.json.JSONEncoder)
* [Sending NaN in json](https://stackoverflow.com/questions/6601812/sending-nan-in-json/6602204)
* [Loop through all nested dictionary values](https://stackoverflow.com/questions/10756427/loop-through-all-nested-dictionary-values) Note: there may be some intricacies in this step, given JSON structure.

Start with discussion #21 and Flask's JSONEncoder documentation, then trace how the API configures JSON responses. Define the expected behavior for NaN and inf, including nested JSON structures, and review the linked references before determining how it should be tested.

Written by the indexing model from the issue text.