Fix bug with param defaults

[gbrammer]

Set default parameter values if missing from the HDF5 file.

(Fixes #52.)

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 62.66%. Comparing base (f1599cf) to head (3c906a6).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
eazy/hdf5.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   62.63%   62.66%   +0.02%     
==========================================
  Files          14       14              
  Lines        5372     5375       +3     
==========================================
+ Hits         3365     3368       +3     
  Misses       2007     2007              

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