Add misp2cef example

[RichieB2B]

A way to export all to_ids attributes in CEF format. Adopted from https://github.com/MISP/misp-modules/blob/master/misp_modules/modules/export_mod/cef_export.py by Hannah Ward.

[codecov]

Codecov Report

Merging #197 into master will decrease coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
- Coverage   64.61%   64.28%   -0.33%     
==========================================
  Files          23       23              
  Lines        3408     3408              
==========================================
- Hits         2202     2191      -11     
- Misses       1206     1217      +11

Impacted Files	Coverage Δ
pymisp/tools/create_misp_object.py	63.51% <0%> (-6.76%)	⬇️
pymisp/tools/machoobject.py	80.59% <0%> (-2.99%)	⬇️
pymisp/tools/elfobject.py	81.42% <0%> (-2.86%)	⬇️
pymisp/tools/peobject.py	79.41% <0%> (-1.97%)	⬇️

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[coveralls]

Coverage decreased (-0.3%) to 64.29% when pulling 7dd2f54 on RichieB2B:misp2cef into 6a3b05f on MISP:master.