Add cmocean v2 :diff, :topo, :tarn and :rain colorschemes.

[evetion]

As released in 2019. Some quick plots with an elevation model in the Netherlands.

topo

diff

tarn

rain

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.14%. Comparing base (543a16c) to head (5fe516b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #144   +/-   ##
=======================================
  Coverage   51.14%   51.14%           
=======================================
  Files           1        1           
  Lines         131      131           
=======================================
  Hits           67       67           
  Misses         64       64           

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[cormullion]

Thanks!