Use an expression for tau-mix rather than a value.
@mahanse made some changes in the matrix assembly stuff that should facilitate this.
There are two (I think) places to pay attention to:
- parse_reactor_parameters in ParseInputFile.cpp. You will need to create the expression for tau_mix there. For now, we can simply support constant expressions. Then the parser won't need to change at all.
- Where the matrix is constructed.
Once this is done, we can remove ReactorEnsembleSpecs and change parse_reactor_parameters to a void
function.