* docs/project.org: suggestions for grader tweaks; start of histogram implementation
* reports.py, calculator_helpers.py: first stab at histograms (UNTESTED!)
* schoolutils/grading/calculator_helpers.py: freqs_for_letters: skip nonexistent grades
* schoolutils/reporting/reports.py: bugfix: histogram bins
* schoolutils/reporting/reports.py: display number and letter for averages
* schoolutils/grading/ui.py: better prompts for edit_table
  - fixes "course #" confusion
* docs/project.org: TODO updates
* schoolutils/reporting/reports.py: bugfix: provide mean_as_letter to formatter
* schoolutils/grading/ui.py: remove dead code: SimpleUI.select_course
* schoolutils/grading/ui.py: support for selecting last due assignment
  - new user option in examples/config.py: use_last_due_assignment
  - grading/ui.py: select last due assignment on startup and on course
    change, depending on user config
* docs/project.org: TODO update