Transmission curves to extrapolate with value 0

[oczoske]

Some SVO filters (e.g. NACO filters) do not drop to zero transmission at the edges of their wavelength range. Scopesim's download_svo_filter used to create synphot SpectralElements with (implicit) fill_value=nan, which means that the filter curves are extrapolated with constant values equal to the edge values, resulting in potentially huge leaks. This PR sets fill_value=0. explicitely, removing those leaks.

This fixes the main discrepancy in AstarVienna/HowManyBloodyPhotons#40.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.87%. Comparing base (c348806) to head (ead783a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #859   +/-   ##
=======================================
  Coverage   74.87%   74.87%           
=======================================
  Files          69       69           
  Lines        8863     8863           
=======================================
  Hits         6636     6636           
  Misses       2227     2227           

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