replaced load and save of solution class with BSON (http://bsonspec.org/, https://github.com/mongodb/mongo-c-driver)

DOFs: 429 087

XML impl.
file size: 38.475 MB
load time: 5552 ms
save time: 6721 ms

BSON impl.
file size: 8.747 MB
load time: 37 ms
save time: 58 ms