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.

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