Introduced in 16b3a43f.

The level that is required by the existing constructor is superfluous.

FTreeCoordinate and FPoint have a lot of methods in common. FTreeCoordinate now
inherits from FPoint to avoid code duplication.

Correct shadow warning.
Change Real typedef to FReal for consistency.
Change operator>> to match orpator<< output.

The timers are mapped to a std::string. The basic ones (P2L, P2M, M2M,
M2P, M2L, L2L, L2P, P2P) have a constexpr definition. Any new timer can
be added by calling `Timers["timer name"];`.

Those fixes are needed when FTic does not rely on OpenMP timer.

Add a peek method to check timer without stopping it
Make tac return elapsed time
Add c++11 std::chrono timing

Those two methods are added to have a consistent naming scheme of the
getters. The previous methods getCenterOfBox and getNbOfParticles still
exist.