Allow (de)serializing CalendarRange and DateRange Parameters

[maximlt]

I got the following error while working on a Lumen app that has a DateRange filter widget synced to the URL:

  File "/Users/mliquet/miniconda3/envs/lumen-dev37/lib/python3.7/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type datetime is not JSON serializable

We found out that the DateRange and CalendarRange parameters inherit from the Tuple parameter (de)serialization methods, while they should override these methods as Python's json module can't serialize datetime objects (import datetime as dt, json; json.dumps(dt.datetime.now()) raises the TypeError mentioned above).

This PR adds these methods to the two parameters, and test them. I've added tests, including tests that use np.datetime64 and pd.Timestamp objects since they appeared to be supported (I learnt that pd.Timestamp objects inherits from datetime.datetime).

[codecov-commenter]

Codecov Report

Merging #625 (ab01157) into master (b38d3b4) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #625      +/-   ##
==========================================
+ Coverage   82.81%   82.89%   +0.08%     
==========================================
  Files           5        5              
  Lines        3037     3070      +33     
==========================================
+ Hits         2515     2545      +30     
- Misses        522      525       +3     

Impacted Files	Coverage Δ
numbergen/__init__.py	80.67% <0.00%> (+0.34%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b38d3b4...ab01157. Read the comment docs.

[jbednar]

Looks good to me!

[maximlt]

Actually any container Parameter that holds a datetime object won't be able to be serialized due to the limitations of the basic Python JSON encoder. As such the changes made in this PR aren't yet enough to fix the issue I observed in Lumen, which uses the DateRangeSlider widget offered by Panel whose value attribute isn't a DateRange parameter but a Tuple.

The way forward may be to subclass json.JSONEncoder and json.JSONDecoder to implement custom encoders/decoders that handle datetime values. There must be some existing implementations of that in the Python ecosystem.

[maximlt]

JSON has no datetime representation so it's actually not possible to directly decode a Python datetime value encoded to a JSON string. Seems like a limitation that should be mentioned in the documentation. The Lumen/Panel issue needs to be fixed separately.

[philippjfr]

This seems sufficient for the specific bug we are fixing here. In the longer run it would be nice to work out some more general solution but I suspect we'd need composite parameter types to make that feasible.