Allow Python interfaces to optionally read from specfile

[jfowkes]

For now just let UGO Python interface read from specfile, more to follow. @AndrewLister-STFC please test.

[codecov-commenter]

Codecov Report

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

Project coverage is 43.38%. Comparing base (d56eb41) to head (5b89529).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #366   +/-   ##
=======================================
  Coverage   43.38%   43.38%           
=======================================
  Files         313      313           
  Lines      161619   161619           
  Branches    55978    55978           
=======================================
+ Hits        70122    70123    +1     
  Misses      73963    73963           
+ Partials    17534    17533    -1     

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