C++ rewrite WIP:

Time_grid structure to Time_grid class
WIP time_gric c++ rewrite.

Constructor almost finished.
WIP spatial_mesh class
Domain class
Field_solver class
Particle and Particle_source classes.
vector<Particle> in particle_source.

And a number of related changes.
std::random instead of gsl_random
Separate class to map particles to mesh.
Command line parser in Boost.
Generation of particles at each time step.
WIP: Multiple sources
Cpp rewrite finished. More or less.

Cout instead of printf.
New feature: mean momentum for source in config.
Merge branch 'cpp' into dev