Add SYCL implementation of LeapFrogGpu

Add SYCL implementation of LeapFrogGpu

An implementation of LeapFrogGpu integrator using SYCL. Also, some additions to SYCL's DeviceBuffer.

Code pretty much follows CUDA version.

For float3, I had to use gmx::RVec instead of SYCL's built-in cl::sycl::float3, since latter is 16 bytes.