Gather/scatter2

[Joroks]

adresses #41. @vchuravy

I've started from scratch with my gather/scatter implementation, because I've overlooked some part of the C-API perviously that allows me to determine the count regardless of what type of data gets gathered/scattered. I therefore also removed count from the parameter list.

[vchuravy]

Thanks! This is a great contribution.

I was wondering what your plans are with LAMMPS.jl
Would something like #28 be helpful in your work?

[Joroks]

You're welcome! I think gather and scatter is basically everything that I need for my project at the moment. I'm currently working on Implementing a Finite-Element-Molecular-Dynamics coupling method, that alternates between FE and MD simulation steps. So after each iteration (either FE or MD) I have to update the positions and forces on certain atoms designated as anchors.

The original implementation of that Method (written in Matlab) used to read/write this information from/to dump files while starting a new LAMMPS instance for each MD iteration step. With this change I don't have to bother with reading/writing files while also hopefully removing the overhead from restarting LAMMPS for each Iteration Step.

I'm hopeing that this will give me a nice performance boost - apart from the ~100x performance increase I was already able to get from writing the FE simulation in Julia (using Ferrite) instead of Matlab.