Adding simple performance test for voxel_mapper with googletest

[orlando21lara]

I simply wrote two tests that just run one function each to measure the time it takes to run them.

Since we want to measure performance, I was considering that we could instead use google/benchmark. Googletest seems to be more suitable for checking the functionality of the code rather than the performance.

[XuRobotics]

@fcladera seems like the checks are failing, should we still go ahead and merge?

[orlando21lara]

Thank you Xu and Chao. I've committed the changes discussed above.

[fcladera]

Follow up in #166