Replaced eval with asteval

[ccomb]

Replaced eval with asteval.

(extracted from #170 )

But still waiting for asteval 0.9.30 with newville/asteval#116 from @newville

[ccomb]

@cmutel asteval 0.9.30 is released, so this PR can be merged

[cmutel]

Thanks @ccomb

• Can you please give a brief justification why we would want to support this more complicated option? I don't see a referenced issue or any text, maybe I just missed it.
• Can you change isinstance(number, (int, float)) to isinstance(number, Number), and add from numbers import Number?
• There are some whitespace or formatting changes included which don't reference the PR content. It would be better if these were removed (they can be submitted as a separate PR, I assume they acutally follow our formatting standards which is a good thing).

[ccomb]

eval is unsafe for arbitrary user input, as reminded by @BenPortner Or maybe you were expecting I replace asteval.Interpreter with bw2parameters.Interpreter and add bw2parameters as a dependency of bw2io ? Please tell

[BenPortner]

@ccomb

Or maybe you were expecting I replace asteval.Interpreter with bw2parameters.Interpreter

Exactly :)

and add bw2parameters as a dependency of bw2io

Yes. In fact, bw2parameters is already an indirect dependency of bw2io via bw2data. However, we should now add bw2parameters as a direct dependency in setup.py and requirements.txt.

[ccomb]

Replaced by #206