Implement unit testing for extra-functionality constraints in the solving script

[ekatef]

Describe the feature you'd like to see

We have a number of functions in solve_network script which implements diverse extra-constraints for the model, while they testing could have been limited up to the data. Examples: add_chp_constraints or add_EQ_constraints

Given the domain significance of these functionality, unit testing can work perfectly there to ensure stability against regression. The implementation approach can contain of the following steps:

• refactoring of the solving scripts
• debug and introducing fixes for all the functions in question
• formulation requirements of UT for each function
• implementation of UT themselves.

[ekatef]

FYI @GbotemiB
May you be interested to formulate the requirements for unit testing? I remember that you have still be testing add_chp_constraints which could give great insights for this task.

As a comment, that is crucial to understand what exactly do we aim to check to maintain the balance between simplicity, ability to capture bugs and stability to false-positive.