give simple_python extra option for meshparty et al

[jefferis]

• navis+fafbseg-py is quite useful but a bit fiddlier to install
• meshparty is a real pain to install
• therefore now have "full" for fafbseg and "extra" for meshparty et al

[codecov[bot]]

Codecov Report

Merging #188 (2e7b2d8) into master (1f714c5) will increase coverage by 0.03%. The diff coverage is 0.00%.

:exclamation: Current head 2e7b2d8 differs from pull request most recent head b48b80c. Consider uploading reports for the commit b48b80c to get more accurate results

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   54.01%   54.04%   +0.03%     
==========================================
  Files          38       38              
  Lines        4719     4720       +1     
==========================================
+ Hits         2549     2551       +2     
+ Misses       2170     2169       -1     

Files	Coverage Δ
R/utils.R	49.37% <0.00%> (-0.13%)	:arrow_down:

... and 1 file with indirect coverage changes

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