Change how averages are represented

[david-pl]

Currently, averages are symbolic functions with a QNumber argument, i.e. avg(::QNumber)::CNumber. Once the metadata system of SymbolicUtils is fixed, we can change that so that averages are directly represented in the format required by ModelingToolkit. We can write them as time-dependent variables x(t) and store the QNumber argument as metadata. This saves us the step of converting to ODESystems with the varmap.