Improve repl

This PR cleans up a bit the BlissRepl code to provide something a bit more maintainable.

I have started a architecture design related to that at https://confluence.esrf.fr/display/BCUKW/BLISS+2.1.x+Architecture, it's a work in progress.

• BlissOutput is now a proxy, no need mixin, nor multiple implementations for each pt output kind.
• BlissRepl is not anymore a Singleton
• There is a one2one relation between BlissSession <-> BlissRepl (in lib mode there will be no BlissRepl obviously)
• ErrorReport have it's own module
• Session._pt_output was removed and now .output can be used
  • (this probably can be read from .bliss_repl but for now i prefer to keep both for now)
• elog_add is not anymore mixed with BlissOutput, now BlissRepl provides an API to read the previous cells

For the next PR i think about

• Restructure a bit the tests
• Simplify some stuffs in the Repl
• Pick some stuffs from #4151 (closed)