Improve APPLgrid/fastNLO converters

[cschwan]

In some datasets several bins are combined together, something that isn't detected in ./appl2pine, see here: https://github.com/NNPDF/fktables/issues/17. This should be either detected automatically or the grid should be written out despite the failing tests.

Furthermore, we need an option to manually specify the alpha order of the process, which isn't stored in the grids.

[cschwan]

Commit 4379f96 adds support for setting the power of alpha, which can be given as an optional third argument. If it's not given it's set to zero (old behaviour).

[cschwan]

The combination of bins cannot reliably detected automatically, so we should do it manually in the one case where it is needed; with commit 424ff5e pineappl sum --bins=from-to <INPUT> <OUTPUT> can be used to do this.