Use absolute path to check if FMU exists.

[AnHeuermann]

Solves errors like

error:   [addSubModel] file does not exist: "..\..\..\..\..\..\..\..\..\fmus\2.0\cs\win64\Dymola\2019FD01\ControlledTemperature\ControlledTemperature.fmu"

on Windows 10 or

error:   [fmiLogger] module FMICAPI: Could not load the FMU binary: File name is too long.
error:   [instantiate] Could not load "resources/0001_fmu.fmu" which is associated with "model.root.fmu"; it may be corrupted or may not support your platform

on Ubuntu with wine. Example is from https://libraries.openmodelica.org/fmi-crosschecking/OMSimulator/history/build-v2.1.1-58%202021-03-25/wine64/files/20.cs.Dymola.2019FD01.ControlledTemperature/20.cs.Dymola.2019FD01.ControlledTemperature.log.html

Related Issues

Partial fix for #789, but if the absolute path is still to long there is not much I can do about it. The function in question is std::filesystem::exists.

Purpose

Fixes

Approach

Convert all paths passed to addSubModel to an absolute path before doing anything with it.

[AnHeuermann]

It could be possible for users to change their Windows to accept longer paths: https://docs.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation?tabs=cmd

But other then that I can't see much more we can do to solve to long names.

[AnHeuermann]

It seems that the test is stuck: https://test.openmodelica.org/jenkins/blue/organizations/jenkins/OMSimulator/detail/PR-1016/7/pipeline

[lochel]

@AnHeuermann please have a look. It's not very sophisticated, but might be just good enough.