Add control function and improve serialization

[1-Bart-1]

closes #87, closes #88 and closes #86

• Add kwargs create_prob::Bool=true, create_lin_prob::Bool=true, create_control_func::Bool=false to init!
• Only deserialize if sam.full_sys is nothing
• Only create the prob, lin_prob or control functions if they don't exist yet in the deserialized model and the corresponding kwarg is true
• Only serialize the model if prob, lin_prob or control functions were added

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.24607% with 32 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/symbolic_awe_model.jl	82.68%	31 Missing ⚠️
src/simulate.jl	0.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!