Allow initialisation of p_inj and q_inj in SP_NetworkInjection

Currently, it is not possible to initialise the active and reactive power of the NetworkInjection class for powerflow simulations.

This could be solved by adding the line:

sol_Q(pv->matrixNodeIndex()) += extnet->attributeTyped<CPS::Real>("q_inj")->get() / mBaseApparentPower;