PLCgen crashes on nested arrays

The following CIF model:

uncontrollable u;

plant a:
  disc list[2] list[3] bool x;
  location:
    initial;
    edge u do x[1][1] := true;
end

Crashes for code generation using default settings of PLCgen:

java.lang.AssertionError
	at org.eclipse.escet.common.java.Assert.check(Assert.java:38)
	at org.eclipse.escet.cif.plcgen.generators.DefaultTypeGenerator.convertType(DefaultTypeGenerator.java:115)
	at org.eclipse.escet.cif.plcgen.generators.DefaultVariableStorage.addStateVariable(DefaultVariableStorage.java:75)
	at org.eclipse.escet.cif.plcgen.targets.PlcBaseTarget.generate(PlcBaseTarget.java:245)
	at org.eclipse.escet.cif.plcgen.CifPlcGenApp.runInternal(CifPlcGenApp.java:167)
	at org.eclipse.escet.common.app.framework.Application.runApplication(Application.java:315)
	at org.eclipse.escet.common.app.framework.Application.run(Application.java:180)
	at org.eclipse.escet.common.eclipse.ui.BaseFileCommandHandler.run(BaseFileCommandHandler.java:261)
	at org.eclipse.escet.common.eclipse.ui.BaseFileCommandHandler$1.run(BaseFileCommandHandler.java:102)
	at java.base/java.lang.Thread.run(Thread.java:840)

It crashes on the following line:

            Assert.check(!(arrayType.getElementType() instanceof ListType)); // TODO Multi-dimensional or nested arrays.

It appears we miss a precondition check. Nested arrays are also not yet listed in the documentation as being not supported.

Addresses #679