BLOCKER: Allow multiple templates per raw data

Multiple fixes in the PR:
- Allows multiple prep templates per raw data
- Preprocessed data is now linked with prep template rather than raw data
- Fixes #326 
- Changes on initialize.sql:
  - SPECTRA removed
  - SFF added
  - FASTA -> FASTA-Sanger

This PR allows @antgonza to continue with his PR.
I was thinking if it should be a classmethod or an instance methos. Sometimes makes more sense to be a instance method, cause you are passing the id. However, that would mean that the user has an object that no longer has a representation on the database, and any further call to it will fail. But this is like pointers on C, if you free them and then try to use them; it's a bug.

What do you think?