[#GH51] Parsing heuristic solutions found prior to nodelog (but not by NoRel)

[venaturum]

• [x] closes #51
• [x] tests added
• [x] tests passed
• [x] all linting tests pass
• [x] example notebook updated
• [x] changelog entry

This adds a new dataframe entry to the timelines dictionary with key "pretreesols".

I don't 100% love this key name, and am open to ideas for changing.