Improve performance of Run.remove_solution()

[glatterf42]

Mainly as a reminder for myself (this is currently an improvement idea for a feature not yet on main, but I don't know if I can address this before the feature gets to main):
#101 (I think) is likely bringing in a function of the sort Run.remove_solution(). If I remember correctly, I implemented this by iterating over all equations and variables etc defined for run x and deleting their levels and marginals (or setting them to 0 or so).
As we heard in this week's performance workshop by @meksor, this kind of iteration is likely slow, so we can probably increase performance of this operation by constructing a query of the sort DELETE equation.data WHERE equation.run__id == x.