Adding extern template statements to speed up compilation

`Softsusy<SoftParsNmssm>` is instantiated in `src/nmssmsoftsusy.cpp`, which is integrated into `libsoft.so`.  Therefore, it is ensured that `Softsusy<SoftParsNmssm>` is available in `libsoft.so` and a further instantiation in `softpoint.cpp` and `main-nmssm.cpp` can be omitted.