add support for hdf5 saving (issue 442)

[ismael-mendoza]

Closes #442

• how to save and load survey objects? add utility functions?
• do we need to interpolate PSF image when loading it or just wrap around galsim.Image()?

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Codecov Report

Patch coverage: 96.22% and project coverage change: +3.05% 🎉

Comparison is base (de35165) 59.47% compared to head (3424000) 62.53%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #448      +/-   ##
==========================================
+ Coverage   59.47%   62.53%   +3.05%     
==========================================
  Files          19       19              
  Lines        1498     1508      +10     
==========================================
+ Hits          891      943      +52     
+ Misses        607      565      -42     

Flag	Coverage Δ
unittests	62.53% <96.22%> (+3.05%)	⬆️

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Files Changed	Coverage Δ
btk/blend_batch.py	72.03% <96.22%> (+19.82%)	⬆️

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