Linear solvers using CUDA do not work or yield wrong results

[LennartSchu]

Describe the bug
When compiling DPsim with CUDA-support (e.g. using the cmake-flag -DWITH_CUDA=ON), the linear solvers implemented in files MNASolverGpuSparse.cpp and MNASolverGpuDense.cpp yield segmentation faults, if an example with a variable system matrix is used, or wrong results if, for example, the model WSCC_9bus_mult_coupled is used - possibly due to the presence of zeros on the system matrix' diagonal. In case of CUDADense, some values are NaN

To Reproduce
Steps to reproduce the behavior:

1. modify example files of WSCC_9bus_mult_coupled and DP_WSCC9bus_SGReducedOrderVBR to use CUDADense (for example)
2. build
   1. mkdir build && cd build
   2. cmake .. -DWITH_CUDA=ON
   3. make WSCC_9bus_mult_coupled DP_WSCC9bus_SGReducedOrderVBR
3. cd dpsim/examples/cxx
4. run
   1. ./WSCC_9bus_mult_coupled -ocopies=0
   2. ./DP_WSCC9bus_SGReducedOrderVBR

Expected behavior
No segmentation faults and correct results, if compared to other solvers, such as EigenSparse