Fix compiler warning in EpwFile.cpp

@jasondegraw @macumber 

I get the following compiler warning and fixed it by adding a 'default' to the switch statement.

openstudiocore/src/utilities/filetypes/EpwFile.cpp:209:10: warning: 
      6 enumeration values not handled in switch: 'Year', 'Month', 'Day'...
      [-Wswitch]
  switch(field.value())
             ^
1 warning generated.