Squashed 'sglib/' changes from b4b5d18..4d3ac10

4d3ac10 applied a proposed fix to apply_boundary_conditions_solution
d254eaa Added an error message to multiindex...

git-subtree-dir: sglib
git-subtree-split: 4d3ac10f84f520a1046c5ecdf3daf285ff2f7f44
Merge commit 'c20384e7b2b30a08d66d5dffa31197ce2ca5e772'