Renamed kw `dims` of `LinearExpression.sum` & `Variable.sum` to `dim`

[leuchtum]

Fixes #245

Release notes need to be changed!

[FabianHofmann]

thanks @leuchtum, I added a release notes and also adjusted the auxiliary function ._sum

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.79%. Comparing base (f182b7f) to head (a17a0aa).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #246      +/-   ##
==========================================
- Coverage   88.88%   85.79%   -3.10%     
==========================================
  Files          15       15              
  Lines        3671     3682      +11     
  Branches      862      866       +4     
==========================================
- Hits         3263     3159     -104     
- Misses        283      407     +124     
+ Partials      125      116       -9     

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[FabianHofmann]

thanks @leuchtum, I will merge as soon as I have some time to change the code in pypsa accordingly (likely beginning of next week)