Moved images folder to the top of the repo

It didn't make any sense that the folder containing our test images would be
located in the HoughSequential-folder while it is actually used by both the
sequential and the parallel version. Therefore, i moved it to the top of the
repo hierarchy.
Fixed segfault, more sanity checks

Fixed a segfault in gpuToCImg(..) caused by passing &cpuData instead of cpuData to cudaMemcpy. The program doesn't crash now, but the gradient strength computation isn't working anyways.

Also added sanity checks for kernel launches and cudaFree's all over the place

Lastly, i added dummy return values for functions that aren't implemented yet.