Speed up writing C3D-files from python

[Kilidsch]

This PR speeds up the c3d.write() call in python. This is done by 2 ways:

1. The filling of the new_c3d-object was moved from Python to C++
2. The Label-Check was impoved ( not by me, but by @pariterre )

The results in the following improvements on c3d.write() on the example file Cortex.c3d` on my machine:

Method	Time in seconds
Original/Old	5.2061723
new_c3d filled in C++	2.2334236
new_c3d filles in C++ and improved Label-Check	0.4963968

So in this particular example we see an 10x speed improvement.

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I don't know if the code changes will be covered by existing tests. They should be though, since writing to disk is no new functionality.

If I deviate from the coding style of the rest of the code base too much, please tell me how to improve.

[pariterre]

LGTM

[codecov]

Codecov Report

Merging #218 (79c890e) into dev (726e4f8) will increase coverage by 0.00%.
The diff coverage is 75.00%.

@@           Coverage Diff           @@
##              dev     #218   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files          22       22           
  Lines        4725     4728    +3     
=======================================
+ Hits         4624     4627    +3     
  Misses        101      101           

Impacted Files	Coverage Δ
src/ezc3d.cpp	96.46% <75.00%> (+0.01%)	⬆️

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[pariterre]

GG!