Allow removing objective expression from the model

[Irieo]

Add functionality to remove a linear expression of the objective from the model.
complements already existing functionality, such as remove_variables() and remove_constraints()
Useful for tutorials, and more generally, for any type of programming exercises when one wants to keep variables and parameters from n.optimize.create_model() but use own objective.

example use:

n.optimize.create_model()
n.model.objective

LinearExpression: +70 Generator-p[now, gas]
Sense: min
Value: None

n.model.remove_objective()

LinearExpression: +0
Sense: min
Value: None

closes #209

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a97febe) 89.26% compared to head (ab97cca) 89.26%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #212   +/-   ##
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  Coverage   89.26%   89.26%           
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  Files          15       15           
  Lines        3352     3354    +2     
  Branches      771      771           
=======================================
+ Hits         2992     2994    +2     
  Misses        248      248           
  Partials      112      112           

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[FabianHofmann]

Awesome @Irieo, to increase the awesomeness, could you add a test for it, probably in test_model.py?

[Irieo]

Awesome @Irieo, to increase the awesomeness, could you add a test for it, probably in test_model.py?

indeed, let's keep the level of awesomeness pumped up. I added a test.