Allow multi-object search

[JulienPeloton]

Closes #291

Now one can query the data for many objects at the same time:

import requests
import pandas as pd
import numpy as np

r = requests.post(
  'https://fink-portal.org/api/v1/objects',
  json={
    'objectId': 'ZTF19acmdpyr,ZTF21aaxtctv',
    'withupperlim': 'True'
  }
)

# Format output in a DataFrame
pdf = pd.read_json(r.content)
n_oids = len(np.unique(pdf.groupby('i:objectId').count()['i:ra']))
# 2 objects

The argument is a comma-separated list of ZTF object IDs.

Todo:

• Update documentation.