write.neurons saves matadata by default

[dokato]

Addresses #483 .

[codecov]

Codecov Report

Merging #499 (b2dd261) into master (1e0a7c0) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #499      +/-   ##
==========================================
- Coverage   75.77%   75.73%   -0.05%     
==========================================
  Files          48       48              
  Lines        5900     5906       +6     
==========================================
+ Hits         4471     4473       +2     
- Misses       1429     1433       +4     

Impacted Files	Coverage Δ
R/neuron-io.R	83.16% <ø> (ø)
R/neuronlist.R	81.30% <0.00%> (-0.87%)	⬇️

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