Improve unit-testing #6
Description
Improve coverage to be above 95%; Ref: andrewrosemberg/HydroPowerModels.jl#13
Some ideas (but any good test is welcome):
- An initial idea could be to add unit tests for functions that create new constraints, variables and add terms to the objective (check if the expression is correct).
- Similar tests to https://github.com/NREL-Sienna/HydroPowerSimulations.jl
- Similar tests to: https://github.com/lanl-ansi/PowerModels.jl
- Add tests to textbook cases.