[task] elaborations on expressions

[tlambert03]

Originally posted by @funkey in https://github.com/funkelab/ilpy/pull/9#pullrequestreview-1357319789

things we could build on top of the expressions:

• [x] conversion of expressions into objectives (see #13)
• [x] support for quadratic objectives (see #15)
• [x] support for quadratic constraints, which are not part of the C++ wrapper, yet (see #12)
• [ ] definitions of ranges (0 <= x <= 10 could be allowed)
• [ ] support for boolean expressions (x > 0 and y == 1 = z)