Use "spawn" instead of "instantiate" when talking about Coral slaves

[kyllingstad]

The current terminology comes from FMI, but this is constantly causing confusion because fmiInstantiateSlave() is in fact not called in Coral's "instantiation" phase. Rather, it is called in the "setup" phase. We should therefore switch to use the word "spawn" instead (better suggestions are welcome) and leave "instantiate" to FMI.