Photochemistry

No comments came so here's the pull request.

The photochemical rate is the integral of the cross-section times the flux, here the multiplication of the bins' height times the bin length.

Photochemistry is added, basically, it bring a VectorCoeffType template needs at a high level (Reaction) kinetics-wise. New objects are:
 - particle_flux.h: stores the abscissa and the flux, general for other kind of reaction to be integrated (electron impact)
 - photochemical_rate.h: stores the cross-section, calculate a rate constant given a particule_flux, update it if necessary. It stores also the cross-section on the particle_flux bins, in case the flux changes but not the bins, only the rate constant needs to be recomputed.
 - sigma_bin_converter.h: capability to change the bin distribution, basically, putting the cross-section on the flux bins

The user should take care of its particle_flux, it contains booleans to see if anything is changed, if the bin distribution is changed, all is recalculated, if only the flux is changed, only the rate constant is calculated.

Have fun!!