Add logical and bitwise operators for math_eval

[Lnaden]

Adds the ability to provide additional operators to the math_eval function. Only adds the and, &, or, and | operators.

This is a feature needed in an upcoming version of YANK

[andrrizzi]

Looks good!

I wonder if we do want to support also and and or because I've noticed now that they are not very intuitive with sets.

>>> a = set([1, 2, 3, 4])
>>> b = set([2, 3, 4, 5])
>>> a and b
{2, 3, 4, 5}
>>> a & b
{2, 3, 4}
>>> b and a
{1, 2, 3, 4}

Sorry if I noticed only now! If we do decide to support them, we definitely need to document this in bold because I believe users will tend to use those instead of & and |.

[andrrizzi]

Can you maybe add a couple of test cases in test_utils.py:test_math_eval before merging?

[Lnaden]

I wonder if we do want to support also and and or because

They are both supported right now but both are set operations (so bitwise) because this is only suppose to be a set operator, not logical operator. But I can make that even more clear now if you want

[andrrizzi]

They are both supported right now but both are set operations (so bitwise) because this is only suppose to be a set operator, not logical operator.

Oh! Sorry I didn't notice that you assigned ast.BitAnd to operator.and_. I understand now.

Feel free to merge! This is not essential at all, but you could also come up with a recursive implementation, instead of unrolling the loop, for consistency with the rest of the function. Something like this may work for example (untested)

elif isinstance(node, ast.BoolOp):
    if len(node.values) > 2:
        # Left-to-right precedence.
        left_value = copy.deepcopy(node)
        left_value.values.pop(-1)
    else:
        left_value = node.values[0]
    return operators[type(node.op)](_math_eval(left_value), _math_eval(node.values[-1]))

[Lnaden]

I liked your solution better, I had tried the recursion but only parsed the values, so it threw an error the first time I tried it. Your's seems to work