Log Message:
google-benchmark: update to 1.6.0.

features

    [breaking change] introduce accessorrs for public data members (#1208)
    add support for percentage units in statistics (#1219)
    introduce coefficient of variation aggregate (#1220)
    format percentages in console reporter (#1221)

bugfixes

    fix unreachable code warning (#1214)
    replace #warning with #pragma message (#1216)
    report PFM as found when it is
    update u-test value expectations due to scipy upgrade

other stuff

    refactored documentation to minimise README.md (#1211)
    install docs when installing library (#1212)