Conflicting inputs should raise an exception

[tadamcz]

IMO, incompatible outputs should raise an exception. Right now:

1. it fails silently
2. the behaviour depends on the order of the kwargs (!)

>>> sigfig.round(12345678, sigfigs=3, notation="scientific", output_type=float)
12300000.0

>>> sigfig.round(12345678, sigfigs=3, output_type=float, notation="scientific")
'1.23E7'

[MikeBusuttil]

I lean more towards alerting the user with a warning instead of raising an exception but certainly the user should be made aware of the conflict.