Feature/longlong particles

Berenger Bramas requested to merge feature/longlong-particles into develop

the particles system is now based on long long int when dealing with number of particles (indexes, size, iteration, etc...). This type can be templatized.

In the main generated by python, the type of "nparticles" is int or long (it is long if there are more than 1<<31 - around 1billion - particles).

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