Add subcommand to modify hadronic initial states

[cschwan]

When calculating the predictions for collisions such as anti-proton with proton, or lead with proton, the generators typically generate grids that must be convoluted with PDFs of the corresponding hadrons. This is disadvantageous if one wants to fit the PDF of a single hadron. Therefore we need a subcommand that transforms the luminosity function and subgrids such that the desired PDF is used for convolution while not changing the predictions.

This can be done easily for anti-particles using charge conjugation, but nuclei more complicated than protons would require more complicated modelling.

[cschwan]

Charge conjugation is implemented in commit c54598cdecb967d1166c124837a77940568c857f.

[cschwan]

Since #299 we can easily do this using Grid::convolutions().