Document expected locations for OspModelDescription.xml

[ljamt]

It is not documented where libcosim looks for the OspModelDescription.xml. It first looks for it in the same directory as the corresponding FMU, if not found it checks the directory of OspSystemStructure.xml.

We should also include information about OspModelDescription.xml and distributed co-simulation, as FMU-proxy does not support OspModelDescription.xml. Keeping the OspModelDescriptions in the configuration directory should still work with FMU-proxy.