Consider improving user accessibility of OA algorithms

[bertcarnell]

From U Groemping:

While looking for OAs for the creation of SOAs, I was thinking about
making your lhs functions for OA creation more visible in DoE.base - I
have incorporated some of the OAs explicitly, but it of course doesn't
make sense to create and store all conceivable maximum size OAs from
your create functions. But it would be nice to have a user-accessible
function that works like DoE.base::show.oas - or even to incorporate
this into show.oas eventually - to which you give your intention, e.g.
20 factors with at least 19 levels each, and receive an answer which
constructions would provide these.