create units_started_up and units_shut_down variables for ramping cases

[nnhjy]

In cases where the user uses the "ramp_up/down_limit" (e.g., issue #1029 ), the ramp_up or ramp_down constraint needs, respectively, the units_started_up or units_shut_down variables. Otherwise, the model may become infeasible. However, the variables are not always activated in such cases unless the user specifies the dedicated parameters.

Checklist before merging

• Documentation is up-to-date
• Unit tests have been added/updated accordingly
• Code has been formatted according to SpineOpt's style
• Unit tests pass