Deprecate `grid_latin_hypercube()` and `grid_max_entropy()`

[hfrick]

With #329 we now have a new grid_space_filling() which is one function for all available types of space-filling designs.

With that we have a mismatch in what's available/exported between different types of space-filling designs though: we have grid_latin_hypercube() and grid_max_entropy() but no, say, grid_audze_eglais().

Thus we deprecate grid_latin_hypercube() and grid_max_entropy(), starting with a soft-deprecation.