Renamed curl computation kernel

CMakeLists.txt

include/libgmshdg/kernels_cpu.h

include/libgmshdg/kernels_gpu.h

src/libgmshdg/kernels_cpu.cpp

src/libgmshdg/kernels_cuda.cu

src/maxwell/maxwell_cpu.cpp

src/maxwell/maxwell_gpu.cpp

tests/test_curl.cpp

tests/test_curl_gpu.cpp

tests/test_differentiation.cpp