Set duration_period in add_horizon()

[francescolovat]

This PR closes #282.

This PR edits the add_horizon method for Scenario class found in /message_ix/core.py

Change of data argument to **kwargs, and added automatic generation of duration_period parameter. Added description in the docstring of add_horizon.

Still, from #282 the following bullet needs to be addressed:

• Clarify and test whether it should be possible to call add_horizon more than once. If so perhaps warn or return a value if the user adds overlapping sets of years; if not, raise an exception.

Please help, @khaeru I don't know why the changes in the core.py file are not reflected in the jupyter notebook files for the tutorials (I was testing my changes in the Westeros tutorial file but nothing was changing when I was editing or even commenting some methods in core.py).

PR checklist:

• Tests added.
• Documentation added.
• Release notes updated.

[khaeru]

Please help, @khaeru I don't know why the changes in the core.py file are not reflected in the jupyter notebook files for the tutorials (I was testing my changes in the Westeros tutorial file but nothing was changing when I was editing or even commenting some methods in core.py).

I guess you were not restarting the kernel or Jupyter—right? Searching for "Jupyter module reload" brings this and this as first results.

[khaeru]

Postponing to after v3.0.

[khaeru]

Per discussion in Slack, I'll jump in here to help complete this.

[codecov]

Codecov Report

Merging #286 into master will increase coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #286      +/-   ##
==========================================
+ Coverage   90.32%   90.46%   +0.13%     
==========================================
  Files          31       31              
  Lines        2740     2780      +40     
==========================================
+ Hits         2475     2515      +40     
  Misses        265      265              

Impacted Files	Coverage Δ
message_ix/message_ix/tests/test_core.py	100.00% <0.00%> (ø)
message_ix/message_ix/core.py	95.18% <0.00%> (+0.81%)	⬆️

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[khaeru]

Thanks @francescolovat!

[francescolovat]

Thank you @khaeru for improving and merging this work! I see that just a little of my code survided to your exhaustive revision 😅