External Torque on Particles

I've added an external torque to the particle options and documented it in the user's guide. It was tested by calculating the rotational diffusion coefficient of a colloidal sphere and the obtained results agreed perfectly with previously established results. 

We've also updated the reaction code to make it more efficient. We've changed the interface and set the reaction up such that its behavior is more in line with that actual chemical reactions. The feature REACTIONS was further renamed to CATALYTIC_REACTIONS. Of course, the User's Guide was completely updated to reflect the changes and the code was extensively debugged.

Be warned, the 'torque' branch was built upon my previous 'galilei_transform' branch, so the commands in the auxiliary.tcl script are still replaced by new  (more appropriately named) commands. However, I have now removed a small bug that occurred in compiling that code without the ROTATION feature. I've also replaced all the instances where the old commands were used in the testsuite and scripts folders, putting the GALILEI feature as a requirement to evaluate those scripts.