add REDSET_ENCODE to select encoding method at runtime

[adammoody]

This modifies redset_init() to read a REDSET_ENCODE environment variable, whose value is used during encode/decode operations to select the acceleration method. This allows one to compile multiple methods and select one at runtime.

Valid options are:

• CPU - main process executes encoding
• OPENMP - main process with OpenMP parallelized for loops
• PTHREADS - use pthreads, where an implementation is available, otherwise fallback to use CPU/OPENMP
• CUDA - use CUDA kernels, requires MPI with GPUDirect RDMA support

The CPU and OPENMP options call the same function. To disable OpenMP threads, one must either build with -DENABLE_OPENMP=OFF or set an OpenMP variable like OMP_NUM_THREADS=1.