Add modelica system cmd (#279)

* [ModelicaSystemCmd] draft

* [ModelicaSystemCmd] define and use it - needs cleanup!

* [ModelicaSystemCmd] update handling of simargs

* [ModelicaSystemCmd] move handling of simflags info this class

* [ModelicaSystemCmd] cleanup / docstrings

* [ModelicaSystemCmd] simplify

* [__init__] make ModelicaSystemCmd available

* [ModelicaSystemCmd] special handling for override in simflags / simargs

* [ModelicaSystemCmd] split run() - create command in get_cmd() and get_exe()

* [ModelicaSystem] use pathlib.Path() / simplify

* [ModelicaSystemCmd] do *NOT* raise error if returncode != 0

could be a simulation which stoped before the final time ...

* [ModelicaSystemCmd.run()] use repr(cmdl) in log messages / exceptions

* [ModelicaSystemCmd] fix exception handling

* define specific exceptions

* [tests] for ModelicaSystemCmd

* [ModelicaSystem] fix flake8 error

./OMPython/ModelicaSystem.py:1236:72: E999 SyntaxError: f-string: unmatched '['

* [ModelicaSystemCmd] spelling fix

* [LinearizationResult] spelling fix

* [ModelicaSystemCmd] update definition of arg_set() - use Optional[]

* [ModelicaSystemCmd] fix mypy warnings

* [ModelicaSystemCmd] some cleanup of docstring for parse_simplags()

* [ModelicaSystemCmd] additional type hints clarifications

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Co-authored-by: Adeel Asghar <[email protected]>

Author: syntron