Processscheduler

New features:
- export to excel

Misc:
- documentation update
- delay solver initialization, better tracking for conflicting constraints
- explicitly raise an exception is ResourceUnavailable defined before the resource is assigned to the tasks. Fix issue 126
- raise exception if resource is identified by its name and not the variable name
- change report status in check_sat
- fix costs computation
- fix boolean examples in features notebook
- move first order logic tests to test_fol.py
- many typos/cosmetic changes

Minor fixes

New features:
- new constraints: OrderedTaskGroup, UnorderedTaskGroup
- add the builtin z3 optimizer as a solver option

Misc:
- max_time can be set to inf(inity)
- fix cost computation
- refactor the Constraint class hierarchy

New features:
- new ResourceTasksDistance constraint
- new NonConcurrentBuffer class
- new TasksContiguous constraint

New features:
- applied Black formatting
- Hotfix: temporal debugging log

New features:
- new incremental solver for optimization
- performance improvements
- new Workload constraint
- single resource flowtime optimization objective
- linear and polynomial cost functions
- add a benchmark folder including one benchmark