Fix threshold for structural variants

[mathieulemieux]

Updating threshold to 48bp for structural variants, as per last clininfo meeting.
Tests don't need update since they are using the constant.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (6e44017) 91.96% compared to head (f639044) 91.96%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #101   +/-   ##
========================================
  Coverage    91.96%   91.96%           
========================================
  Files            8        8           
  Lines          834      834           
========================================
  Hits           767      767           
  Misses          67       67           

Flag	Coverage Δ
unittests	91.96% <100.00%> (ø)

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Files Changed	Coverage Δ
graphkb/constants.py	93.33% <100.00%> (ø)

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