ENH: changes in objective::write

- Expanded the write function in the base class so that it can manage
  input coming from the derived ones. This reduces a lot of code
  duplication in the latter but keeps the functionality.
- Added a default width for all entries in the objective files.
- If a normalisation factor or a target is set, they are written on the
  header of the objective file.
- Cosmetic/code consistency changes in various files.