Add validation for min_up_time and min_down_time

[datejada]

Add a default value for min_up_time and min_down_time if the online_variable_type is binary or integer.

Fixes #960

Checklist before merging

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