Feature/make preferred gene source configurable

[elewis2]

adds optional preferred-source arg to functions that call get_preferred_gene_name. This is because the value used is an rid which will be different in different instances of the db. To make this function usable with a non-bcgsc pori instance, the value needs to be settable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f2bc298) 90.35% compared to head (d520623) 90.35%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #114   +/-   ##
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  Coverage    90.35%   90.35%           
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  Files            8        8           
  Lines          840      840           
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  Hits           759      759           
  Misses          81       81           

Flag	Coverage Δ
unittests	90.35% <100.00%> (ø)

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