Rework ModelicaSystem.simulate

[lafrech]

Follow-up to #214. See comments.

Popen should be fed a list of arguments, not a single string. The fix in #214 splits the string but

• arguments should be split one step further ("-r path" -> ["-r", "path"]
• it makes no sense to concatenate strings then split them, the arguments should be built as a list in the first place