Add CUDA version of 'radial'

[drroe]

Version 6.11.0.

Adds a GPU version of the radial command using CUDA. Currently this is only for atom-atom RDFs (one or two masks). The speedup can be significant. Here's a comparison of the previous OpenMP code (run on an Intel sandybridge, 4 cores) to the new CUDA code (run on a GTX 780):

  CPPTRAJ BENCH (3): Radial solvent-solvent benchmark.
    TOP: ../FtuFabI.tip3p.parm7  CRD: ../FtuFabI.tip3p.nc  NTRAJIN: 1
    TRAJIN_ARGS: 1 10
	/home/droe/Cpptraj/rdfgpu.cpptraj/bin/cpptraj.OMP
	Iter   Time Context_Switches  Waits      FPS
	0     91.12             5982     36    0.110
	1     91.26             6146     36    0.110
	2     91.22             7455     36    0.110
	/home/droe/Cpptraj/rdfgpu.cpptraj/bin/cpptraj.cuda
	Iter   Time Context_Switches  Waits      FPS
	0      1.26                1     92   12.518
	1      1.26               21     89   12.527
	2      1.23               22     85   12.506
  Baseline  :  91.2000
  Benchmark :   1.2500
  Speedup   :  72.96
  %Change   :  98.63 %

The GPU RDF is calculated in single precision, so small numerical differences on the order of 0.0002-0.0004 can be expected in the resulting histograms.

This PR also includes a 2x speedup in the CPU code when only a single mask is specified (e.g. when calculating water oxygens to water oxygens RDF).