Add new conditional cmake flag to enable specification of cuda architecture at build time

[AndrewBMadison]

Add a conditional flag to set alternate cuda architecture in build.

The cache variable TARGET_ARCH can be passed in on the command line to change the cuda architecture used.

e.g. when running the GPU simulation on target architecture 70:
cmake -D ENABLE_CUDA=YES -D TARGET_ARCH=70 ..

The quickstart guide also needs to be updated for this change.