noteweb: Rendering bird field notes for the Web

Each instance of this class holds all the information we need to build one month's cell in the index table.

# - - - - -   c l a s s   M o n t h C e l l

class MonthCell:
    '''Represents one month's entry in the index table.

      Exports:
        MonthCell ( yearRow, mm, birdNoteSet ):
          [ (yearRow is the containing YearRow instance) and
            (mm is the month as a left-zero-filled string 'mm') and
            (birdNoteSet is a birdnotes.BirdNoteSet instance
            representing that month's data) ->
              return a new MonthCell instance with those values ]
        .yearRow:        [ as passed to constructor, read-only ]
        .mm:             [ as passed to constructor, read-only ]
        .yyyy_mm:        [ (yearRow.yyyy)+'-'+(mm)
        .title:          [ self's 'monthName yyyy' string ]
        .birdNoteSet:    [ as passed to constructor, read-only ]
        .fileName():
          [ return the path name of the month's file relative
            to the index page ]
        .writePage():
          [ if self's XHTML page can be created anew ->
              that page  :=  self rendered as XHTML ]
    '''