More sophisticated outcome constraints

[thopkins32]

Objective constraints are currently limited to fixed ranges but we can use Ax to allow for more flexible outcome constraints.

This includes:

• Constraints that combine multiple objective values (e.g. obj1 + obj2 <= 200)
• Constraints measured against baseline readings (e.g. obj1 <= 5 + baseline). See Attaching baseline readings #127 for more information on baseline readings.

[thopkins32]

Simplest thing to do is to allow setting constraints explicitly using strings rather than tying them to specific Objectives. Not sure if it makes sense to keep constraints in Objectives after this. We should deprecate, if so.