ModelicaSystem - use OMCPath

[syntron]

A big step on the way to run ModelicaSystem not only locally; see #315 item (1):

1. need of file system interactions => solution: create OMCPath, an implementation of a pathlib.Path object which uses OMC as backend (see detailed analyses which functionality is needed in PR [ModelicaSystem] only local omc process allowed #309)

based on PR #317, #314, #320 and #321

addditional requirements: PR #329 and PR #330