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).