Add dims argument to LogSumExpAtom

[odow]

Closes #682
Closes #685

[odow]

So perhaps the simplest thing is just if we add a ; dims argument to relevant of atoms.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.21%. Comparing base (7a8c0df) to head (bae4c9a).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #692      +/-   ##
==========================================
+ Coverage   98.20%   98.21%   +0.01%     
==========================================
  Files          89       89              
  Lines        5168     5198      +30     
==========================================
+ Hits         5075     5105      +30     
  Misses         93       93              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ericphanson]

Lgtm, nice and simple. I think it would be worth carrying over the tests in src/problem_depot/problems/exp.jl from the other PR