Slow import

[jwittenbach]

Running import polyseq is a bit slow. This is likely due to a combination of two factors:

1. all imports are done at the top of each module
2. we do a lot of import chaining in the __init__.py files to get nice autocompletion when traversing the package structure.

Taken together, this means that we are probably importing just about every dependency that we have on the initial import polyseq. One work-around, that would maintain the nice autocompletion, would be to move some of the more specific imports inside the functions that use them.