Remove all warnings with gcc 8: memset et memcopy une now reinterpret_cay<char*> and we remove restrict fir target and source in p2P

Now we use scaoed enum for OPENMP priorities ; Rename typed particles (source, target) rather than FParticleSource

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

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

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.

The classes FMpiBufferWriter and FBufferWriter implemented the same
functionality, FMpiBufferReader and FBufferReader did too. This replaces
the FBuffer implementations with the FMpiBuffer ones.

Steps taken:
  - remove FBuffer* implementation
  - rename FMpiBuffer* to FBuffer*
  - clean up code and documentation
  - fix missing includes

A few unused variables warnings are fixed in FAbstractBuffer.hpp.

There was a discrepancy in the getSize method of F(Mpi)BufferReader. The
behaviour of FMpiBufferReader is kept and the unit test is modified.

The classes FMpiBufferWriter and FBufferWriter implemented the same
functionality, FMpiBufferReader and FBufferReader did too. This replaces
the FBuffer implementations with the FMpiBuffer ones.

Steps taken:
  - remove FBuffer* implementation
  - rename FMpiBuffer* to FBuffer*
  - clean up code and documentation
  - fix missing includes

A few unused variables warnings are fixed in FAbstractBuffer.hpp.

There was a discrepancy in the getSize method of F(Mpi)BufferReader. The
behaviour of FMpiBufferReader is kept and the unit test is modified.

Variable was given instead of its address.

The multipole and local expansion share the same implementation. Their
types only differ by a template tag parameter. Consquently, the
`getMultipole`/`getLocal` methods are changed to `get`.

The method change is propageted to the files:

  - FChebKernel,
  - FChebSymKernel,
  - FChebTensorialKernel,
  - FAdaptChebKernel,
  - FUnifKernel,
  - FUnifTensorialKernel,
  - FAdaptUnifKernel,
  - utestChebyshevDirectPeriodic,
  - utestInterpolationMultiRhs,

Change relevant call to getMultipoleData and getLocalExpansionData.