Change modifications stored in database

[ruiwanguk]

For better support in search and PRIDE Proteome, we need to perform the following tasks:

• Sort modifications by position
  • Change the importer code to sort
• Standardise the modifications using Yasset's modification library
  • Change the database schema to add one extra column
  • Change the importer code to use Yasset's mod library
  • Change the repo code to expose the standardised modification