COMP: Upgraded GoogleTest to 1.8.1
COMP: Upgraded GoogleTest to 1.8.1

Upgraded GoogleTest to 1.8.1, to fix VS2017 build errors:
error C2220 warning treated as error: no 'object' file generated [SuperElastix-build\Testing\googletest-build\googlemock\gmock.vcxproj] [F:\X\Bin\vc141-64\SelxDev\D\SuperElastix-build\Testing\googletest-download\googletest.vcxproj] SuperElastix-build\Testing\googletest-src\googletest\include\gtest\internal\gtest-port.h

gtest 1.8.1 is from August 2018 (whereas gtest 1.8.0 was from July 2016).

SuperElastix/SuperElastix,SuperElastix/SuperElastix,kaspermarstal/SuperElastix,kaspermarstal/SuperElastix,SuperElastix/SuperElastix,kaspermarstal/SuperElastix,SuperElastix/SuperElastix