A benchmark for the PCP tree

[pleroy]

No optimization so far. It demonstrates that there is a sweet spot in the number of values per node for the query. Related to #3358.

The numbers:

Run on (4 X 3310 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x4)
  L1 Instruction 32 KiB (x4)
  L2 Unified 256 KiB (x4)
  L3 Unified 6144 KiB (x1)
--------------------------------------------------------------------------------
Benchmark                                      Time             CPU   Iterations
--------------------------------------------------------------------------------
BM_PCPBuildTree/1000/1                   1742994 ns      1737205 ns          449
BM_PCPBuildTree/1000/4                    703417 ns       695192 ns         1122
BM_PCPBuildTree/1000/16                   387528 ns       383746 ns         1870
BM_PCPBuildTree/1000/64                   248415 ns       250353 ns         2991
BM_PCPBuildTree/1000/256                  184105 ns       183580 ns         3739
BM_PCPBuildTree/10000/1                 18981213 ns     18973095 ns           37
BM_PCPBuildTree/10000/4                 10173109 ns      9984064 ns           75
BM_PCPBuildTree/10000/16                 5766233 ns      5710751 ns          112
BM_PCPBuildTree/10000/64                 4062164 ns      3967655 ns          173
BM_PCPBuildTree/10000/256                3333376 ns      3288256 ns          204
BM_PCPBuildTree/100000/1               190323815 ns    182001167 ns            3
BM_PCPBuildTree/100000/4               112673797 ns    114400733 ns            6
BM_PCPBuildTree/100000/16               67603615 ns     65236782 ns           11
BM_PCPBuildTree/100000/64               53451681 ns     54600350 ns           10
BM_PCPBuildTree/100000/256              45305502 ns     44964994 ns           17
BM_PCPFindNearestNeighbour/1000/1           2060 ns         2034 ns       345165
BM_PCPFindNearestNeighbour/1000/4           1098 ns         1095 ns       641022
BM_PCPFindNearestNeighbour/1000/16           754 ns          730 ns       897430
BM_PCPFindNearestNeighbour/1000/64           713 ns          723 ns      1121788
BM_PCPFindNearestNeighbour/1000/256         1214 ns         1217 ns       641022
BM_PCPFindNearestNeighbour/10000/1          5000 ns         5148 ns       100000
BM_PCPFindNearestNeighbour/10000/4          3192 ns         3171 ns       236166
BM_PCPFindNearestNeighbour/10000/16         1921 ns         1874 ns       407923
BM_PCPFindNearestNeighbour/10000/64         1372 ns         1363 ns       560894
BM_PCPFindNearestNeighbour/10000/256        1628 ns         1599 ns       448715
BM_PCPFindNearestNeighbour/100000/1        29201 ns        28439 ns       897430
BM_PCPFindNearestNeighbour/100000/4        15000 ns        14115 ns       897430
BM_PCPFindNearestNeighbour/100000/16        6248 ns         6202 ns      1121788
BM_PCPFindNearestNeighbour/100000/64        4096 ns         4068 ns       897430
BM_PCPFindNearestNeighbour/100000/256       4506 ns         4422 ns       373929