(Closes #2585) Add an optional, 'default' argument to SymbolTable.lookup()

[arporter]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.86%. Comparing base (4560b8b) to head (06b841f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2645   +/-   ##
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  Coverage   99.86%   99.86%           
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  Files         352      352           
  Lines       48509    48512    +3     
=======================================
+ Hits        48444    48447    +3     
  Misses         65       65           

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[arporter]

Just adds an optional, 'default' argument to the lookup() method. One for @sergisiso, @hiker or @LonelyCat124. (Dev. Guide description of lookup has updated automatically.)

[arporter]

Dev. guide still builds fine. CI happiness permitting, this is ready for another look.

[arporter]

CI-happiness permitting, this is ready for another look.