This allows loading factorizing code between the FCountKernel and other
kernels for generating or loading particles from a file.

The container expects that the particle exposes a method `as_tuple` that
converts the particle object to an std::tuple.

The container no longer hides the underlying container value_type. It
provides a particle_t alias to the particle type.

FTree will deduce its particle type from the container in two steps:
  - if the container exposes a particle_t type, it is used
  - otherwise, the value_type alias of the container is used

The macro NDEBUG is undefined in the tests

A define is added to choose the adaptive of the uniform tree.

This allows altering the LICENCE text without changing all files. It should
avoid merge conflicts in the future.

Better consistency with rest of code.

This generates less classes and allows some overloading that did not
work for const iterators.

The accessors were references to private members, they are changed to
standard member function accessors.

Significant modif in Chebyshev FMM (may impact your library): Provide a variant of ChebM2L without compression (new name FCheb) and use prefix FChebCmp for the compressed variant (former FCheb); Now FChebTensorial does exactly the same thing as FCheb but for tensorial kernel matrices.

Uniform Cube, Ellipsoid and Plummer can be generated (default is Uniform
Cube).

Add define to use file or not in testBlockedMpiChebyshev.cpp testBlockedMpiUniform.cpp and testFmmAlgorithmThreadProc.cpp