Allow combining `notation` keyword with `output_type=str`

[tadamcz]

The docs say that notation

should not be used in conjunction with kwarg format/style or type/output_type (since that would essentially be asking for conflicting outputs).

However, when output_type=str, it's not obvious to me what the conflict is. And the current behaviour is just confusing:

>>> sigfig.round(66698127389217, sigfigs=3, notation="scientific")
'6.67E13'

>>> sigfig.round(66698127389217, sigfigs=3, notation="scientific", output_type=str)
'66700000000000'

[tadamcz]

I just realised the behaviour depends on the order of the kwargs, last kwarg wins. I think that's also highly counter-intuitive, raised a separate issue: #13

So in summary:

1. I think it would be good to add more argument checking, so that conflicting inputs raise an exception.
2. AFAICT, notation=sci (or similar) together with output_type=str should not be considered a conflict.

[MikeBusuttil]

Thanks for this @tadamcz
I agree, the argument parsing portion of this library certainly needs some love