Resolve "Support proposal sessions"

Closes #2694 (closed) #3243 (closed)

Adapt to changes in the ESRF data policy:

• The root path of visitor proposals now has this template:

  /data/visitor/{proposal_name}/{beamline}/{proposal_session_name}

• Experiments IH* and BLC* become visitor proposals from a directory pov. The only inhouse proposals left are proposal names that start with the beamline name (e.g. the default inhouse proposal which is selected when typing newproposal()).

• The newproposal method gets two new (optional) arguments:

  newproposal('blc123')  # automatic selection of the proposal session
  newproposal('blc123', session_name="20210614")  # manual selection of the proposal session
  newproposal('blc123', prompt=True)  # prompt for selection of the proposal session

  If no session name is provided and prompt=True then a dialog will be shown which allows you to select one of the possible session names (if more than one exists). The list of possible session names is created by inspecting the proposal directory.

• Bliss validates a session name (provided by manual/prompt selection or None) in this order:

  1. If the provided session_name is already equal to the current session name: keep it
  2. If the provided session_name is in the list of possible session names: take it
  3. If the current session name is in the list of possible session names: keep it
  4. If there is only one possible session name: take it
  5. If there is more than one possible session name: take the one with the last start date before the current date
  6. Choose the default session name "yyyymm01" (first of the month)

  The list of possible session names is created by inspecting the proposal directory.