Turn the library_id back into the primary key for samples_library (SCHEMA CHANGE!)

Turn the library_id back into the primary key for samples_library (SCHEMA CHANGE!)
Trying to make it possible to enter the 'library_id' instead of the
arbitrary auto-incrementing key when creating a flowcell was turning out
to be far too time consuming.

It was vastly easier to decide that the 'library id' was a sufficiently
unique short value that it could be used directly as the primary key.
Its now a char 10 field, unlike the integer primary key to support
stanford style library IDs like SL123.

Its possible to convert the previous database version to one compatible with
this code by running docs/conv_library_id_to_pk_v0.3.1.py