Add subcommand to sum bins

[cschwan]

If we want to generate a 2D distribution from 3D one, for instance, we need a subcommand to sum over several bins:

• add switch --integrated to produce integrated cross sections; implemented in commit ad80e83
• add switch --sum-obs to integrate over the dependence of entire observables (changes N-dimensional grids to (N-1)-dimensional ones)
• add switch --bins to sum individual bins; implemented in commit 424ff5e

[cschwan]

All of this is now possible with pineappl write --merge-bins, although 1. one has to specify the indices manually and 2. this is possibly subject to this bug: #125.