Make cs optional for import. Refs #100

[koenedaele]

No description provided.

[coveralls]

Coverage: 99.281% (+0.01%) from 99.266% when pulling a2c4c3e on feature/100_conceptscheme_optional into ccfa585 on develop.

[coveralls]

Coverage: 99.283% (+0.02%) from 99.266% when pulling 2a0b7cf on feature/100_conceptscheme_optional into ccfa585 on develop.

[koenedaele]

@Wim-De-Clercq I've had to change the method signature to make the ConceptScheme optional, so this would a BC breaking change. I don't think there's a good way of avoiding that?

[Wim-De-Clercq]

I don't see a good way of keeping it BC, the only way would be to have a function with a new name and keep the old one.

fyi, Python does have some "overloading" capabilities. BUT they call it 'dispatching' and in the standard library it only works on the first argument https://docs.python.org/3/library/functools.html#functools.singledispatch

Of course we could download a third party library which works on more arguments, but that starts to feel like overkill.