Introduce run.get_by_id()

[glatterf42]

The second part of cleaning up #101. This exposes the run.get_by_id() function to all remaining data layers as it is only used by run.clone() (see next clean up PR) in the data/server layers (and present already in the DB layer).

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.0%. Comparing base (b0201c6) to head (8513cac).

Files with missing lines	Patch %	Lines
ixmp4/data/abstract/run.py	50.0%	1 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           enh/run-remove-solution    #140   +/-   ##
=======================================================
  Coverage                     87.0%   87.0%           
=======================================================
  Files                          230     230           
  Lines                         8162    8170    +8     
=======================================================
+ Hits                          7101    7113   +12     
+ Misses                        1061    1057    -4     

Files with missing lines	Coverage Δ
ixmp4/data/api/run.py	97.9% <100.0%> (+0.1%)	⬆️
ixmp4/server/rest/run.py	100.0% <100.0%> (ø)
ixmp4/data/abstract/run.py	80.8% <50.0%> (-1.4%)	⬇️

... and 2 files with indirect coverage changes

[meksor]

lgtm