Simplify max_as/l application

[alecandido]

When max_as/l are specified, to use grids at a lower order than available (e.g. computing an NLO theory from NNLO grids), everything might be done consistently just cutting the grids during loading: everything else (checks and operations) will be done adaptively according to the loaded grids, that will always be used completely.

This will have the advantage of only loading (or keeping in memory) the required subgrids, lowering the memory impact of Pineko in these cases, and the even greater advantage of simplifying the code, since there will be no need to separately consider the case of grids containing orders higher than needed.