Decrease size of downloadable moderna package.

[lenarother]

Some ideas in #56.
Ask users about opinion?

[lenarother]

Idea from PP (#56): download single pdb files on the fly. I will gladly try this in a separate branch. KR says the test for lir should find more severe problems. The workflow could be: When fragment is needed download the structure form pdb or rnadb (I would go for pdb as it is more standard) and tun clean structure.

Adventages:

• no 100 pdb files in repo
• initial moderna download is smaller

Disadvantages:

• the Internet access required.
• fragment insertion slower: each time download and clean

[cosi1]

Let me deal with these disadvantages :-)

the Internet access required.

For those who don't have a reliable Internet access (or simply don't want to rely on it), there should be an option to download the whole library at once. If it were a distinct GitHub repository, downloading and installing it locally would be as easy as cloning a repo.

fragment insertion slower: each time download and clean

Not each time. What I meant was that the already used fragments would be left on the disk (if they have been used once, they will probably be of use in the future). Thus the size of ModeRNA will be increasing, but not at once and probably not to the size of the whole library.