This function checks if the variables and constants that are defined in the
experiment are valid.
It loads the model code of the NetLogo model and checks if these variables
and constants really exist.
In case of nonvalid entries, the function throws an error message, indicating
which variables and constants are not valid.
Please note, that this function might fail if the supported modelpath does
not point to an existing nlogo file.
This might for example happen, if the modelpath is set up for a remote
cluster execution.