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

Major modification! Defined rigoreous target-source convention: p(x)=sum_y K(x,y)w(y) with x=target and y=source. Direct computation is done by calling MutualParticles(Target params,Source params) or P2P(Target, Source); Kernel matrices evaluation is done by calling evaluate(x=target,y=source) which returns k(x,y)=k(x-y).

Remove some warnings, and split P2PR which is P2P only for one over R and P2P which can accept a matrix kernel (which can be one over R or anything else), both approaches support SSE and AVX but have not been tested.

Provided generic functions for the evaluation of the direct interactions (pass matrix kernel as argument).

In utestRotationDirectSeveralTime we have to implement a method to reset the leaves to their initial state.

Implemented dense tensorial Chebyshev kernel + add test, fixed some failing utests by comparing RELATIVE errors instead of absolute.

Introduced tensorial physical value, potential and force (through templatization of P2PParticleContainer with dimensions NRHS and NLHS, default NRHS=NLHS=1). ChebTensorial kernel does not support this yet (TODO apply same modif on ChebInterpolator).