Strip empty `(muf2, mur2)` entries in subgrids with dynamic scales

[cschwan]

When we generate grids using https://github.com/NNPDF/runcards with a proper dynamic scale choice - that means a scale prescription that has more than one value for each subgrid - we keep the full scale information in the optimized subgrids. This is not a problem as far as storage is concerned, but we can not see which scale values are 'empty' and which are 'filled'. This is a problem

• if we want to reconstruct the dynamic scale choice (see for instance Add ATLASZPT8TEV{M,Y}DIST datasets pinecards#61 (comment)) through its range: a grid generated with the default parameters will always show all 40 grid points in muf as ranging from 10 GeV to 10 TeV.
• It will also potentially and unnecessarily slow down the evolution of those grids.

[cschwan]

Implemented in commit a80529b for the conversion of LagrangeSubgridV2 to ImportOnlySubgridV1.