Add a types module

[mferrera]

Typing certain parameters could be made simpler with a central types module that describes longer unioned types used widely across the codebase. In particular this includes things like

FileLike = Union[str, pathlib.Path, io.BytesIO, io.StringIO
FileFormat = Literal[...]  # roff, roffbin, grid, grdecl...

However this kind of file can become unwieldy without some discipline. It could, for example, but prudent to graduate types to a global file as we go along, first putting them in deeper modules:

# xtgeo.grid3d.types or _types
GridLike = Union[Grid, GridProperty]

[mferrera]

Resolved with #958