Update developer guide and class diagrams

[peienlim]

• Updated Model and Storage class diagrams
• Included a new proposed event switcher feature under Implementations section

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.54%. Comparing base (7db997e) to head (02a3491).

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #44   +/-   ##
=========================================
  Coverage     69.54%   69.54%           
  Complexity      471      471           
=========================================
  Files            81       81           
  Lines          1609     1609           
  Branches        165      165           
=========================================
  Hits           1119     1119           
  Misses          447      447           
  Partials         43       43           

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[peterXGD]

igtm