upgrade to general expression only when necessary

[fserra]

Closes #213

[ramabile]

Misc:

• Exception TypeError: "'NoneType' object is not callable" in 'pyscipopt.scip.Model.__dealloc__' ignored each time I run tests. Do you know what raises it?
• I should make test_logical.py more compact...

[ramabile]

Disclaimer: I am a chemist by background and I could be biased. I work with classical a-toms, that means integers cannot be divided. 😆

About the request, I think the expected behaviour as a Python user follows (#213):

float / float = float
float / int = float
int / float = float
>>> int / int = int

Since there is no support for int/int I would expect ProdExpr is kept unchanged and not casted as Expr. Doing the opposite opens the pitfall so that an int/int is assumed to be float.
My newcomer's opinion is I would like to face the error as soon as possible instead of discovering by myself integer division is not supported.

What is SCIP behaviour in this case?

[fserra]

I am not sure why the error at __dealloc__ appears. I guess we still have some issue with when the freeing of what happens.

I would say int/int is float. For instance, x/2 and x/2.0 should both be expressions, right?

[ramabile]

Ok I see now, it makes sense.

Python3 makes 3/2==1.5, Python2 3/2==1. There is no support for 3//2.
If one wants the floor division, they realises soon x//2 is unsuported and finds a workaround like this.