igraph dependency now optional

[jajcayn]

• divide requirements to optional and basic
• add notes to README
• import igraph within plotting function (lazy import) instead of a global one
• when igraph not installed, skip ternary tree plotting test
• bump version to 0.3

[codecov]

Codecov Report

Merging #17 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #17   +/-   ##
=======================================
  Coverage   96.06%   96.06%           
=======================================
  Files           5        5           
  Lines         585      585           
=======================================
  Hits          562      562           
  Misses         23       23

Impacted Files	Coverage Δ
gpso/plotting.py	93.9% <100%> (ø)	⬆️

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