[solvers.hh] Always convert the reduction to double explicitly.

This already happened for most solvers when setting the reduction in the result struct. It is may be needed when the field type is some extended number type that prohibits implicit conversion to double. This commit brings the remaining two solvers in line that did not do this yet.

enabled an automatic merge when the pipeline for 82476058 succeeds

merged

mentioned in commit e8223df8

mentioned in commit 4bfa7ec3

mentioned in commit e4911db9

mentioned in merge request !141 (merged)