Replace @kw_only_meth/func decorator with "*" argument

[rmarkello]

Python 3 supports the use of * in function signatures to denote that subsequent arguments must be supplied as keywords. Historically, nibabel has used nibabel.keyword_only.kw_only_{meth,func} to do this, instead. Since nibabel is no longer supporting Python 2 it might be nice to remove these and just use the '*' syntax! I'd be happy to make a PR for this, if you think it would be desirable.

[effigies]

Yup, this would be great.