Cox problem, Many well problem, stability improvements

Various improvements including:

• Adding the 1600 dim cox problem
• Improving general evaluation
• Adding gmm_v1
• Adding better eval for Many Well problem
• Adding flexible point_is_valid_fn
• Bounding real_nvp scaling (greatly improves training stability)
• Switching from haiku to flax

init

v0.0.0

feat: fix blacjax version for now