Make destructors in polymorph classes virtual

[akohlmey]

Some compilers don't like when destructors in polymorph classes are not virtual and complain about it.
This change silences those warnings.

[codecov-commenter]

Codecov Report

Merging #4 (985a26d) into master (ad8e1ea) will decrease coverage by 0.05%.
The diff coverage is 36.84%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
- Coverage   72.27%   72.21%   -0.06%     
==========================================
  Files         130      133       +3     
  Lines       14234    14252      +18     
==========================================
+ Hits        10287    10292       +5     
- Misses       3947     3960      +13     

Flag	Coverage Δ
cpp	74.99% <36.84%> (-0.07%)	⬇️
python	72.21% <36.84%> (-0.06%)	⬇️

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Impacted Files	Coverage Δ
src/libnnp/Mode.h	66.66% <0.00%> (ø)
src/libnnpif/LAMMPS/InterfaceLammps.h	0.00% <0.00%> (ø)
src/libnnptrain/Training.cpp	73.20% <37.50%> (-0.37%)	⬇️
src/libnnp/Settings.cpp	94.19% <100.00%> (+0.02%)	⬆️
src/libnnp/Prediction.h	0.00% <0.00%> (ø)
src/libnnp/Structure.h	0.00% <0.00%> (ø)

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