Add `store_ts` reporting computation

[khaeru]

store_ts accepts pd.DataFrame or pyam.IamDataFrame as input and stores the contents using TimeSeries.add_timeseries().

Also:

• TimeSeries.url (Scenario.url) is a shorthand for constructing the URL of an object.
• Cleaner output from ixmp solve CLI when the target Scenario is not found.

How to review

• Read the diff for computations.py and the added tests.
• Note that the CI checks all pass.

PR checklist

• [x] Continuous integration checks all ✅

• [x] Add or expand tests; coverage checks both ✅
• [x] Add, expand, or update documentation.

• [x] Update release notes.

[codecov[bot]]

Codecov Report

Merging #444 (e99458d) into main (fad98c3) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #444   +/-   ##
=====================================
  Coverage   98.5%   98.5%           
=====================================
  Files         42      42           
  Lines       4382    4424   +42     
=====================================
+ Hits        4320    4362   +42     
  Misses        62      62           

Impacted Files	Coverage Δ
ixmp/cli.py	100.0% <100.0%> (ø)
ixmp/core/timeseries.py	98.7% <100.0%> (+<0.1%)	:arrow_up:
ixmp/reporting/computations.py	100.0% <100.0%> (ø)
ixmp/reporting/reporter.py	100.0% <100.0%> (ø)
ixmp/tests/reporting/test_computations.py	100.0% <100.0%> (ø)
ixmp/tests/test_cli.py	100.0% <100.0%> (ø)
ixmp/utils.py	99.5% <100.0%> (ø)