Support `Parameter` dependencies in .min and .max

It is very natural for users to want a `Parameter` to always be larger than a different `Parameter`. This would naturally be done by setting e.g.
```
a=Parameter(name='a',value=1)
b=Parameter(name='b',value=2,min=a)
```
If a user (or a fit) then updates `a`, the min of `b` updates to the new value of `a`, If `a.value>b.value`, then `b.value=a.value` should be set.