Enable merging bins of higher-dimensional distributions

[cschwan]

See also https://github.com/NNPDF/fktables/issues/26#issuecomment-1067877787.

[alecandido]

Possibly related:

I wonder if it is consistent that the remapper is never being updated during Grid::merge, while Grid.bin_limits is:

pineappl/pineappl/src/grid.rs

Lines 871 to 872 in ff68f36

	self.bin_limits = BinLimits::new((0..=b).map(f64::from).collect());
	other.bin_limits = BinLimits::new((a..=b).map(f64::from).collect());