Function object for adding a human-readable output variable
Since human-readable outvars do not necessarily have any reference to the depvar of their system they refer to, nor which system they refer to, we configure them for all of the systems they are preceded by. If there is only a single system of the type the outvar is configured, we simply look up the depvar and use that as OutVar::var. If there are multiple systems configured upstream to which the outvar could refer to, we configure an outvar for all systems configured, and postfix the human-readable OutVar::name with '_' + depvar. Hence this function object so the code below can be invoked for all equation types.