Add Beacon oscillator

[theodumont]

Please let me know is everything is ok!

[codecov]

Codecov Report

Merging #40 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   97.68%   97.76%   +0.07%     
==========================================
  Files          12       12              
  Lines         259      268       +9     
==========================================
+ Hits          253      262       +9     
  Misses          6        6              

Impacted Files	Coverage Δ
seagull/lifeforms/__init__.py	100.00% <100.00%> (ø)
seagull/lifeforms/oscillators.py	100.00% <100.00%> (ø)

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[ljvmiranda921]

LGTM! Thanks so much and congrats to your first contribution in Seagull! Hope to see you again soon :)