Call direct trafos automatically for smaller sizes.

[jenskeiner]

The library provides (asymptotically) slow direct transforms as well as (asymptotically) faster transforms. The slower transforms may actually be more efficient for smaller problem sizes.

The direct transforms should be invoked automatically instead of the faster ones when the problem size is below a certain threshold; see #34.

[tvolkmer]

The problem of segfaults / buffer overruns was treated in #34. Since this is just a speed issue here, this may be solved in 4.0 or later.