Added Radiation.h file
Added new radiation files for tree Monte-Carlo
1. - Fixed problems with circular dependencies for new radiation classes
2. - Tidied up some parts of the comments
1) Fixed parallelisation problems in BruteForce
2) Fixed allocation memory problem for column format
1) Fixed new Monte Carlo radiation tree for basic propagation of photon packets
2) Added gather search for points
1. - Fixed problem with new gather neighbour search routine
2. - Cleaned up SphParticle.h
Splitting up SphNeighbourSearch routines for each SPH implementation (Part 1; compiles but not complete yet)
1. - Finished splitting SphNeighbourSearch class between different SPH simulations
2. - Fixed small problem with gravity neighbours for direct sum particles
Collection of small changes
1. - Added develops section to user guide
2. - Fixed examples in both examples directory and user-guide
Merge branch 'radiation' into refactoring