Setting a default max/min bound when calling MixedIntegerLinearProgram.new_variable

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Being able to write something like :

```
p = MixedIntegerLinearProgram()
v = p.new_variable(min=3, max =8)
```

Would be really nice !

CC:  @yuan-zhou

Component: **linear programming**

_Issue created by migration from https://trac.sagemath.org/ticket/7790_

