Minor optimizations to the computation of the force.

Before:
```
+// BM_SolarSystem        29550253368 29499789100          1                                 1.0002759262591376e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem        29623949266 29562189500          1                                 1.0002759262591376e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem        29542950130 29484189000          1                                 1.0002759262591376e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem        29571937299 29515389200          1                                 1.0002759262591376e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem        29341910577 29328188000          1                                 1.0002759262591376e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem_mean   29526200128 29477948960          1                                 1.0002759262591376e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem_stddev   96403882   79300082          0                                 1.0002759262591376e+000 ua    // NOLINT(whitespace/line_length)

+// BM_CLR_SimulateSolarSystem        31866679646 31855404200          1
 +// BM_CLR_SimulateSolarSystem        33116308190 32978611400          1
 +// BM_CLR_SimulateSolarSystem        33756355263 33290613400          1
 +// BM_CLR_SimulateSolarSystem        32331227518 32245406700          1
 +// BM_CLR_SimulateSolarSystem        33322325445 33321813600          1
 +// BM_CLR_SimulateSolarSystem_mean   32878579212 32738369860          1
 +// BM_CLR_SimulateSolarSystem_stddev  685193577  587492393          0
```

```
+// BM_SolarSystem        24890476919 24882159500          1                                 1.0002759262590839e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem        26308986163 26301768600          1                                 1.0002759262590839e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem        24715988136 24710558400          1                                 1.0002759262590839e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem        24851988054 24850959300          1                                 1.0002759262590839e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem        25200986600 24975760100          1                                 1.0002759262590839e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem_mean   25193685174 25144241180          1                                 1.0002759262590839e+000 ua  // NOLINT(whitespace/line_length)
 +// BM_SolarSystem_stddev  579799316  584985030          0                                 1.0002759262590839e+000 ua    // NOLINT(whitespace/line_length)

+// BM_CLR_SimulateSolarSystem        31282008148 31278200500          1
 +// BM_CLR_SimulateSolarSystem        32160996176 32136206000          1
 +// BM_CLR_SimulateSolarSystem        32204995848 32198606400          1
 +// BM_CLR_SimulateSolarSystem        32454995132 32448208000          1
 +// BM_CLR_SimulateSolarSystem        31222993589 31168999800          1
 +// BM_CLR_SimulateSolarSystem_mean   31865197779 31846044140          1
 +// BM_CLR_SimulateSolarSystem_stddev  510559831  519986693          0
```
`const`