Teach write.neuron(s) to put metadata in SWC header

[jefferis]

• this could be supported for other formats
• maybe it would be better not to convert to JSON immediately in write.neuron since other file formats might want store metadata differently
• note that we convert data.frame->list so that we dont have a list wrapper around the json

[codecov[bot]]

Codecov Report

Merging #480 (e79f833) into master (a66a1df) will decrease coverage by 0.15%. The diff coverage is 44.00%.

:exclamation: Current head e79f833 differs from pull request most recent head fd2cbb1. Consider uploading reports for the commit fd2cbb1 to get more accurate results

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
- Coverage   76.95%   76.80%   -0.16%     
==========================================
  Files          47       47              
  Lines        5842     5863      +21     
==========================================
+ Hits         4496     4503       +7     
- Misses       1346     1360      +14     

Impacted Files	Coverage Δ
R/neuron-io.R	85.15% <44.00%> (-3.00%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update a66a1df...fd2cbb1. Read the comment docs.

[jefferis]

@alexanderbates (@schlegelp FYI) here is a first WIP version of the code to put metadata into the header of SWC files. You can do

write.neurons(nl, metadata=T)

to use each row of the attached dataframe of neuronlist.