Allow manual definition of test set

[singraber]

• Adds optional keyword after "begin" in input.data
• Can be either "set=train" or "set=test"

[codecov-commenter]

Codecov Report

Merging #175 (383197e) into master (6be249e) will decrease coverage by 0.03%. The diff coverage is 46.66%.

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
- Coverage   74.94%   74.90%   -0.04%     
==========================================
  Files         103      103              
  Lines       13949    13961      +12     
==========================================
+ Hits        10454    10458       +4     
- Misses       3495     3503       +8     

Flag	Coverage Δ
cpp	74.95% <46.66%> (-0.04%)	:arrow_down:
python	72.69% <ø> (ø)

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Impacted Files	Coverage Δ
src/libnnp/Structure.cpp	78.90% <14.28%> (-0.90%)	:arrow_down:
src/libnnptrain/Training.cpp	77.09% <75.00%> (-0.05%)	:arrow_down:

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